Vegan Rhubarb and Custard Blondies

Want soft vegan blondies with sharp rhubarb, a proper custard layer, and edges that do not collapse? Use a thick custard, remove excess rhubarb juice, and chill the bake before slicing. This recipe makes 16 squares in a 20cm square tin, with a crisp top and fudgy centre.

Why vegan rhubarb custard blondies need a firm base

What gives a blondie its chewy centre

Traditional blondies rely on butter, eggs, flour, and brown sugar. The butter adds richness, the sugar holds moisture, and the egg helps the middle set. A vegan version needs structure from several smaller choices: melted plant butter for fat, ground flaxseed or apple purée for binding, and enough flour to support the wet custard.

Use 180g plain flour and 50g ground almonds for this recipe. The almonds soften the crumb without making it cakey. Choose light brown sugar rather than white sugar because its moisture helps create the chewy texture blondies are known for. Mix only until the flour disappears. Extra stirring develops gluten and turns the bars firm.

How custard changes the bake

Custard adds water, starch, and fat to an already soft mixture. Pouring thin custard straight over the batter creates a wet stripe that may never set. Cook the custard until it leaves a clear track when you drag a spoon through it, then cool it for at least 15 minutes before adding it to the tin.

The best target is thick but spreadable custard. It should sit on the batter instead of sinking through it. A chilled blondie will always look firmer than a warm one, so judge the final texture after two hours in the refrigerator.

Why rhubarb must lose some water

Rhubarb releases a surprising amount of juice as it heats. Toss 250g chopped rhubarb with 30g sugar and roast it for 12 minutes at 190°C. Drain the liquid before using the fruit. The pieces should be tender but still hold their shape.

Best texture rule: thick custard, drained rhubarb, and a full cooling period matter more than any egg substitute.

Ingredients, swaps, and the quantities that work

Measure the flour and sugar with a digital scale. Cup measurements vary too much for a recipe with two wet layers. Line a 20cm square tin with baking paper, leaving paper over two sides so the cold blondies lift out without cracking.

Which supermarket brands work

Flora Plant B+tter gives a reliable fat level and melts evenly. Bird’s Original Custard Powder makes a firm dairy-free custard when prepared with plant milk. Alpro Soya Single works well when you need a smooth pourable custard.

How to keep the recipe nut-free

Replace the 50g ground almonds with 50g plain flour. The blondies will have a slightly firmer bite, so add 1 teaspoon of extra plant milk only if the batter looks stiff and dry. Do not replace the almonds with coconut flour; it absorbs far more liquid and can produce crumbly squares.

For the plant milk, Oatly Barista oat drink gives a full texture. Unsweetened soya milk also sets well because it contains more protein. Avoid sweetened rice milk here because it can make the custard loose and overly sweet.

How to make the blondies step by step

Prepare the fruit and custard first

1. Heat the oven to 190°C conventional or 170°C fan. Line a 20cm square tin.
2. Chop 250g rhubarb into 1cm pieces. Toss with 30g sugar and roast for 12 minutes. Drain and cool.
3. Whisk 25g custard powder, 30g caster sugar, and 250ml plant milk in a saucepan while cold.
4. Cook over medium heat for 3 to 5 minutes, whisking constantly, until thick. Stir in 1 teaspoon vanilla and cool until warm, not hot.

Mix and layer the batter

1. Melt 140g vegan butter and cool it for 5 minutes. Whisk with 150g light brown sugar, 100g unsweetened apple purée, and 2 teaspoons vanilla.
2. Fold in 180g plain flour, 50g ground almonds, 1½ teaspoons baking powder, and ½ teaspoon salt.
3. Spread two-thirds of the batter into the tin. Add the cooled rhubarb, then spoon over 180g thick custard.
4. Dollop the remaining batter across the top. Drag a blunt knife through the surface three times to create shallow swirls.

Bake, cool, and slice

Bake for 38 to 45 minutes. The edges should look set, while the centre should wobble slightly when you move the tin. Cool in the tin for 30 minutes, then refrigerate for at least 2 hours. Cut into 16 squares with a warm, dry knife. This timing gives the cleanest slices.

How do you stop the custard from disappearing?

Why did the custard sink?

The custard was probably too thin or too hot. A hot mixture loosens the blondie batter and sinks through it. Cook it until thick enough to mound on a spoon, then wait until it feels warm rather than steaming before layering it.

Can you swirl the custard deeply?

Keep the knife near the surface. Three shallow passes create visible ribbons without cutting through the batter. Deep swirling mixes the custard with the raw flour layer, which produces damp pockets after baking.

Can you use ready-made vegan custard?

Yes, but reduce it first if it pours like milk. Simmer 250g ready-made custard for 2 to 4 minutes, stirring constantly, then cool it. The best ready-made option is one that sits in a spoonful rather than running straight off it.

Clear verdict: homemade thick custard gives the most reliable middle. Ready-made custard saves time only when you reduce it before layering.

Five mistakes that cause gummy or dry blondies

Fix the common failures

• Gummy centre: the rhubarb released too much liquid, the custard was thin, or the blondies were sliced warm. Drain the fruit and chill the finished bake.
• Dry edges: the tin was smaller than 20cm, the oven ran hot, or the bake went past 45 minutes. Check at 38 minutes and remove the tin when the centre still has a small wobble.
• Greasy batter: the melted butter was hot when mixed with the sugar and apple purée. Let it cool until warm before combining.
• Collapsed middle: the blondies were removed before the centre set. Leave them in the switched-off oven for 5 minutes with the door open if the middle looks liquid.
• Hard rhubarb pieces: the stalks were cut too thick. Keep pieces close to 1cm and roast them before they touch the batter.

When not to make this recipe

Do not bake these bars when you need a dessert that can be served immediately. They need cooling and chilling to become sliceable. For a warm pudding, use the same rhubarb and custard flavours in a crumble instead.

How to store the finished squares

Store the blondies in a sealed container in the refrigerator for up to 4 days. Place baking paper between layers. Let one square stand at room temperature for 15 minutes before serving; this softens the butter and brings back the custard flavour without making the centre runny.

Which variation gives the best result?

Choose the fruit by the texture you want

Rhubarb gives the strongest contrast because its sharpness cuts through the sweet blondie. Strawberries make a softer, sweeter bar but release more water, so roast them for 8 minutes and drain them well. Raspberries need no pre-cooking, but they create pockets of juice and a more fragile slice.

Choose the custard by the flavour you want

Soya custard gives the firmest set and a neutral taste. Oat custard tastes rounder and sweeter but may need an extra 5g of custard powder. Coconut custard works with lime zest, though its flavour can overpower delicate rhubarb. For the classic bakery-style result, choose soya custard with vanilla.

Choose the tin by the serving style

A 20cm square tin gives tall, fudgy bars. A 23cm square tin gives thinner blondies that bake in roughly 30 to 35 minutes. Do not use a loaf tin; the deep centre will set slowly while the outside dries. For neat party portions, the 20cm tin is the clear winner.

The final choice: fudgy bars or a warm rhubarb pudding

My recommendation

For sliceable vegan rhubarb and custard blondies, use the 20cm tin, thick homemade custard, roasted rhubarb, and a full two-hour chill. That combination gives the strongest texture contrast: chewy brown-sugar blondie, soft custard ribbon, and bright fruit.

For a quick dessert, skip the blondie batter and make rhubarb crumble with vegan custard. For a lunchbox bake, reduce the custard to 150g and chill overnight for firmer squares. For the best Sunday dessert, serve one cold square with warm custard poured around it.