Vegan Lemon Custard Slice: 12 Zesty No-Bake Squares

If you love the bright, sharp comfort of a classic lemon slice but need it completely plant-based, this vegan lemon custard slice is the recipe to keep. It has everything the traditional version promises — a buttery-tasting crumb base that presses together in minutes, and a smooth, glossy lemon layer that slices cleanly — yet it contains no eggs, no butter and no milk at all. The lemon layer sets thanks to a gentle partnership of cornflour and agar, and it tastes vividly of lemon rather than of its setting agents.

This no-bake vegan lemon custard slice is ideal for warm weather. The base is crushed vegan biscuits bound with melted coconut oil, pressed into a tin and chilled until firm. The custard cooks briefly on the hob — just long enough to thicken and activate the agar — then pours over the base and chills until set. A pinch of turmeric gives the layer its sunny colour without a trace of earthy flavour. The result: twelve neat squares with a sharp lemon bite, a creamy middle, and a base that holds together when you pick it up.

Recipe at a Glance

• Prep time: 25 minutes
• Cook time: 8 minutes (custard on the hob)
• Chill time: at least 4 hours 30 minutes in total (30 minutes for the base, 4 hours for the custard layer) — overnight is even better
• Total time: about 5 hours, mostly hands-off chilling
• Yield: about 12 slices
• Difficulty: easy — no baking, basic hob work only

Ingredients

For the crumb base

• 250 g / 8¾ oz vegan digestive-style biscuits (about 2½ cups of fine crumbs) — check the packet, as some brands contain milk
• 100 g / 3½ oz coconut oil (about ½ cup), melted — or the same weight of a firm dairy-free baking spread, melted
• 1 tablespoon maple syrup or golden syrup (optional, helps the base bind with very dry biscuits)
• A pinch of fine sea salt

For the lemon custard layer

• 400 ml / 14 fl oz coconut milk (1 standard can, 1¾ cups) — full-fat, not the light version
• 200 ml / 7 fl oz coconut cream (scant 1 cup) — or the thick top from a second chilled can of coconut milk
• 120 ml / 4 fl oz fresh lemon juice (½ cup) — from about 3 large lemons
• Finely grated zest of 2 lemons (about 2 teaspoons)
• 100 ml / 3½ fl oz maple syrup (⅓ cup plus 1 tablespoon) — or 110 g / ½ cup golden syrup for a more traditional flavour
• 40 g cornflour / cornstarch (⅓ cup)
• 1½ teaspoons agar agar powder (not flakes — see the notes below)
• ¼ teaspoon ground turmeric, for colour
• ½ teaspoon vanilla extract
• A pinch of fine sea salt

To finish

• 1 tablespoon icing sugar / powdered sugar, for dusting (optional)
• Extra lemon zest or thin lemon slices, to decorate (optional)

A note on agar: powder and flakes are not interchangeable by volume. This recipe uses powder. If you only have flakes, use 1½ tablespoons instead and simmer the custard for a full 5 minutes, as flakes dissolve more slowly. Do not use gelatine — it is not vegan.

Equipment

• 20 cm / 8 inch square baking tin
• Baking parchment / parchment paper, for lining
• Food processor, or a sturdy bag and rolling pin, for crushing the biscuits
• Medium saucepan and a heatproof whisk
• Fine grater or zester, and a citrus juicer
• Mixing bowls, measuring jugs, cups and spoons (or kitchen scales)
• Palette knife or the back of a spoon, for smoothing

Method, Step by Step

Step 1 — Line the tin

Line the square tin with baking parchment, leaving an overhang on two sides so you can lift the finished slice out easily. A dab of coconut oil in the corners helps the paper stay put.

Step 2 — Make the crumb base

Blitz the biscuits to fine crumbs — a few tiny rubbly pieces are fine, but avoid large chunks, which make the base crumble when sliced. Without a processor, put the biscuits in a sealed bag and bash with a rolling pin until fine. Stir in the salt, pour over the melted coconut oil (and syrup, if using) and mix until every crumb looks evenly damp, like wet sand. Tip into the lined tin and press down very firmly and evenly with the base of a glass, paying attention to the corners. Chill for at least 30 minutes, until the base feels hard when tapped.

Step 3 — Mix the custard base (off the heat)

In the saucepan — off the heat — whisk the cornflour and agar powder with about 100 ml / ½ cup of the coconut milk until completely smooth. This slurry step matters: cornflour added straight to hot liquid turns lumpy instantly. Then whisk in the remaining coconut milk, the coconut cream, maple syrup, turmeric, vanilla and salt. Stir in the lemon zest now, but keep the juice aside — adding it after cooking protects its fresh flavour.

Step 4 — Cook and thicken the custard

Place the pan over a medium heat and bring the mixture to a gentle boil, whisking constantly and scraping the corners. Keep it at a steady, bubbling simmer for 2 full minutes, still whisking. This step is essential: agar only activates properly at boiling point, and cornflour needs a minute at the boil to lose its raw, floury taste. The custard will thicken noticeably — it should coat the back of a spoon thickly and leave a clear trail when you run a finger through it. Take the pan off the heat, wait 30 seconds, then whisk in the lemon juice until glossy and even.

Step 5 — Pour, smooth and chill

Pour the hot custard slowly over the chilled base, tilting the tin so it flows into the corners. Smooth the top with a palette knife and pop any large bubbles with a knife tip — small ones disappear as it sets. Leave the tin at room temperature for 20 minutes (this avoids condensation dripping onto the surface), then cover loosely and chill for at least 4 hours, preferably overnight. Be honest with the timing: agar feels firm quickly, but the layer needs the full chill to slice into clean squares rather than soft wedges.

Step 6 — Slice and serve

Lift the slice out using the parchment overhang and set it on a board. For the neatest squares, dip a large sharp knife in hot water, wipe it dry, and cut with one firm downward press rather than a sawing motion; clean and re-warm the knife between cuts. Cut into 12 squares (three rows by four). Dust with icing sugar if you like, add a little fresh zest, and serve chilled.

Tips for a Perfect Vegan Lemon Custard Slice

• Boil the custard properly. Under-boiled agar is the number one reason a slice like this stays soft. The mixture must reach a real, bubbling boil and stay there for the full 2 minutes.
• Measure agar carefully. Too little and the layer slumps; too much and it turns rubbery. Level teaspoons of powder (1½ teaspoons is about 4 g) are the sweet spot for this volume of liquid.
• Press the base harder than you think. A loosely pressed base is the other classic failure. Spend a full minute compacting it with a flat-based glass, especially at the edges.
• Use full-fat coconut milk. Light coconut milk is mostly water; the custard tastes thin and sets less firmly. Shake the can well before measuring.
• Fresh lemon juice only. Bottled juice tastes dull next to fresh, and this slice lives or dies on sharp lemon flavour. Roll lemons firmly on the worktop before juicing to get more juice out.
• Chill overnight for events. For guests, make it the day before — the cuts are at their cleanest.

Variations and Substitutions

• Lime or passion fruit: Swap the lemon juice and zest for the same amount of lime, or use 100 ml / ½ cup passion fruit juice plus 2 tablespoons of lemon juice. Keep the setting agents exactly the same.
• Ginger base: Add ½ teaspoon of ground ginger to the crumbs, or use vegan ginger biscuits for half the base. Ginger and lemon are natural partners.
• Berry-topped slice: Scatter a thin layer of raspberries or blueberries over the base before pouring on the custard; the fruit suspends as it sets and looks beautiful when sliced. A fruity drink alongside works well too — try this raspberry peach smoothie.
• Nutty base: Replace 50 g / ½ cup of the crumbs with finely ground almonds or desiccated coconut; add an extra teaspoon of coconut oil if the mixture looks dry.
• No agar available? Increase the cornflour to 55 g / ½ cup and simmer for 3 minutes. The set will be softer — slice it cold, straight from the fridge, and serve on plates.
• Lower sugar: Reduce the maple syrup to 75 ml / ¼ cup for a sharper, more grown-up lemon hit. Do not cut it further, or the acid overwhelms the custard.

More ideas: try our no-churn banana pudding ice cream for another chilled dessert, or this cranberry orange cake for another citrus bake.

Storage and Make-Ahead

Store the slice in an airtight container in the fridge, in a single layer or separated with parchment, for up to 5 days. The base softens slightly over time; the lemon flavour stays bright. This vegan lemon custard slice is an excellent make-ahead dessert: it genuinely needs its long chill, so making it a day ahead is a help rather than a compromise.

Freezing works, too. Freeze the cut squares on a lined tray until solid, then transfer to a container with parchment between layers for up to 2 months. Thaw squares in the fridge for about 2 hours before serving; the custard stays smooth, though the base may be a touch softer. For general guidance on chilling and storing custard-style desserts safely, see the official US cold food storage chart.

FAQs

Why did my lemon layer not set?

Almost always, the custard did not boil long enough, or agar flakes were measured as if they were powder. Bring the mixture to a genuine rolling boil for the full 2 minutes, and check your agar’s form before you start. A too-soft slice can sometimes be rescued: scrape the layer back into a pan, re-boil for 2 minutes, re-pour and chill again.

Can I bake this slice instead of chilling it?

This vegan lemon custard slice is designed as a no-bake recipe; baking would melt the coconut-oil base and dull the fresh lemon flavour. Chilling gives the cleanest set and the brightest taste.

Does the slice taste of coconut or turmeric?

Only faintly, if at all. The lemon dominates; the coconut milk simply provides the creamy body you would normally get from dairy, and the turmeric is there purely for colour, well below the level where it adds flavour. If you are very sensitive to coconut, use a neutral oat cream instead of the coconut cream and a mild dairy-free spread in the base.

What biscuits are actually vegan?

Many plain digestive and rich-tea style biscuits are accidentally vegan, but recipes change, so always check the ingredients for milk, whey, butter or honey. Vegan graham-style crackers and oat biscuits also work well. Avoid filled or chocolate-coated biscuits — their coatings make the crumb greasy.

Can I make this in a round tin?

Yes — a 20 cm / 8 inch round springform tin works; cut 12 wedges instead of squares.

Helpful Tools

You probably own most of what this recipe needs, but these four make this vegan lemon custard slice noticeably easier:

A square tin with a loose base or springform sides releases the chilled slice cleanly.

Proper agar powder is the key to a firm, clean-cutting set in vegan custards and jellies.

A sharp microplane zester takes only the fragrant yellow zest and leaves the bitter white pith behind.

A heat-resistant silicone whisk thickens custard without scratching your pan and reaches right into the corners.

Final Thoughts

Sharp, creamy and completely plant-based, this vegan lemon custard slice proves a set lemon layer never needed eggs — just the right setting agents, a proper boil, and a patient chill. Make it the day before, cut it with a hot knife, and watch the squares disappear. If you enjoy make-ahead desserts, this chocolate magic custard cake is a lovely next project.