These rhubarb and custard blondies are irresistibly buttery and indulgent. Soft and gooey in the centre with crisp, chewy edges, they are finished with a rhubarb compote, smooth custard and a white chocolate drizzle. The recipe comes together quickly and requires no special equipment—perfect for sharing with friends and family.
Why you will love this recipe
The pairing of tart rhubarb and sweet custard is a classic for a reason. The contrast of textures—tender, jammy rhubarb and silky custard—creates a bright, comforting flavour profile that lifts the rich, buttery blondie base. These bars are dense, fudgy and satisfying, making them a great choice when you want something easier than a cake but more exciting than a simple cookie.
They hold together well, slice neatly after chilling, and the marbling of compote and custard gives each piece a pretty, homemade appearance that’s ideal for gatherings or packed lunches.
Key ingredients and substitutions
This recipe uses pantry-friendly ingredients. Below are the main components and simple swaps you can make if needed.
Rhubarb – Fresh rhubarb is ideal, but frozen or canned rhubarb will also work (drain canned rhubarb first). Rhubarb benefits from some added sugar to soften its tartness; make only as much sweetener as you prefer and save any extra compote for yogurt, porridge or on toast.
Custard – This recipe uses custard powder in the batter to add a soft, vanilla flavour and uses ready-made or prepared custard on top. If you prefer, you can use powdered custard for both the batter and topping—prepare the topping according to the powder instructions and chill before swirling.
White chocolate – White chocolate chips, chopped bars or wafers are all fine. The white chocolate adds sweetness and a creamy texture; reserve some for drizzling after the blondies have cooled.
Sugar – A mix of light brown soft sugar and caster (granulated) sugar gives depth and keeps the texture tender. Brown sugar contributes a gentle caramel note while caster sugar helps the blondies set properly.
Eggs – The recipe works well with either three medium eggs or two large eggs. Use what you have; the texture will remain fudgy if you follow the bake time guidance below.
How to make these blondies
Begin by making the rhubarb compote. Chop fresh rhubarb into 2–3 cm pieces and place in a wide saucepan with caster sugar and 2 tablespoons of water. Cook over medium heat, stirring frequently, until the rhubarb breaks down and becomes jammy—about 8–12 minutes. Allow the compote to cool completely. If you prefer a more vivid colour, a couple of drops of pink food colouring can be added, but this is optional.
If you are making custard from powder for the topping, prepare it now and cool it so it’s ready to dollop on the batter later.
To make the blondie batter, melt the butter with both sugars in a medium pan over low heat, stirring until smooth and glossy. Remove from the heat and allow to cool for about 10 minutes so it’s warm but not hot. In a separate bowl, sift together the flour, custard powder and baking powder.
Beat the eggs into the cooled butter and sugar mixture, then fold in the dry ingredients until you have a smooth batter. Stir in 150 g of the white chocolate and the vanilla extract, then pour the batter into a prepared 8 x 8 inch (20 x 20 cm) tin lined with baking parchment.
Drop spoonfuls of rhubarb compote across the surface of the batter, then add dollops of custard. Use a skewer or toothpick to swirl to create a marbled effect. If using an 8 x 8 inch tin, bake at 180°C (160°C fan) for 45–50 minutes, or until the edges have set and the centre still has some moist crumbs on a skewer. For a fudgier texture remove the blondies when a skewer comes out with slight crumbs attached; for a cakier result bake 5–10 minutes longer.
If you bake in a larger, shallower 20 x 30 cm (8 x 12 inch) tin, expect a shorter bake time of about 35–40 minutes for a fudgy texture and a further 5–10 minutes for a more cake-like result. If the top is browning too quickly, cover loosely with foil. Allow the blondies to cool in the tin, then chill in the fridge for at least two hours or overnight. This resting time is important for the bars to firm up and slice cleanly.
Storing
Store cooled blondies in an airtight container in the fridge for 3–4 days. They also freeze well—wrap individual slices tightly in clingfilm, place in a freezer-safe container and freeze for up to three months. Defrost overnight in the fridge before serving.
FAQs
Can I replace rhubarb?
Yes. Tart fruits like strawberries, raspberries, cranberries or sour cherries work well, as do tart jams. Choose ingredients that will balance the sweetness of the blondie and the white chocolate.
Can I use a different tin size?
Yes. Smaller, deeper tins will need a longer bake time; larger, shallower tins will bake quicker. Adjust the time and use the skewer test to check for desired doneness.
Why do my blondies look under-baked?
Blondies are best removed from the oven when they are slightly under-baked because they will continue to set while cooling. This keeps them fudgy. If you prefer a drier, cakier texture, bake a few minutes longer.
My blondies turned out dry—what happened?
They were likely over-baked. Reduce the bake time next time and use the toothpick test; the skewer should come out with some moist crumbs for a fudgy result.
📖 Recipe
Rhubarb and custard blondies
Equipment
- 8 x 8 inch baking tin
- Baking parchment
- Rubber spatula
Ingredients
- 225 g butter, plus extra for the tin
- 200 g light brown soft sugar
- 100 g caster sugar
- 150 g plain (all-purpose) flour
- 50 g custard powder
- ½ tsp baking powder
- 3 medium eggs
- 250 g white chocolate chips or finely chopped white chocolate (total)
- 2 tsp vanilla extract
For the rhubarb and custard topping:
- 200 g rhubarb, chopped into 3–4 cm pieces
- 75 g caster sugar
- 4 tbsp ready-made custard, or prepare from custard powder
Instructions
- Place the chopped rhubarb into a wide pan with 75 g caster sugar and 2 tablespoons of water. Cook on medium heat, stirring frequently, until the rhubarb breaks down and becomes jammy. Set aside to cool. If using powdered custard for the topping, prepare and cool it now.
- Heat the oven to 180°C (160°C fan). Place the butter and both sugars in a medium pan over low heat and melt together until smooth and glossy. Remove from the heat and cool for about 10 minutes.
- In a large bowl, sift together the flour, custard powder and baking powder. Butter the baking tin and line with baking parchment.
- Beat the eggs into the cooled butter and sugar mixture, then fold in the dry ingredients until smooth. Stir in 150 g of white chocolate and the vanilla. Pour the batter into the tin and use a teaspoon to add spoonfuls of rhubarb compote across the surface.
- Add dollops of custard, then swirl a skewer or toothpick through the rhubarb and custard to create a marbled pattern. Bake for 40–50 minutes until set and the edges are coming away from the tin. If the top begins to brown too much, cover with foil. Leave in the tin to cool, then transfer to the fridge for a minimum of 2 hours or overnight.
- Melt the remaining 100 g white chocolate and drizzle over the chilled blondies. Cut into squares and serve.
Recipes are developed and tested using metric measurements; a digital scale is recommended for best results.
Notes
- You can add a few drops of pink food colouring to the rhubarb compote for a more vibrant colour.
- Frozen or canned rhubarb can be used; drain canned rhubarb first.
- Swap rhubarb for tart fruits or your favourite jam if you prefer a different flavour.
- Store leftovers in an airtight container in the fridge for up to 4 days. To freeze, wrap each slice individually and freeze for up to 3 months; thaw overnight in the fridge.
Nutrition
This post was originally published in February 2020 and has been updated with new photos and tips.