Place the rhubarb and sugar in a wide pan, then add 2 tablespoons of water.
Cook over a low-medium heat, stirring frequently, for about 10 minutes, until the rhubarb breaks down and turns jammy.
Add a few drops of food colouring if you want a pink rhubarb swirl.
Remove from the heat and leave to cool.
Heat the oven to 180°C/gas mark 4.
Place the butter, brown sugar, and castor sugar in a pan, then set it over a low heat.
Melt together until smooth and shiny.
Remove from the heat, then leave to cool for 10 minutes while you sift the flour, custard powder, and baking powder into a bowl.
Butter a 20cm x 30cm baking tin, then line with baking parchment.
Beat the eggs with the cooled butter and sugar mixture, then fold in the dry ingredients until you have a smooth batter.
Stir in 150g of the white chocolate chips, and the vanilla.
Pour the mixture into the prepared tin, then use a teaspoon to swirl the rhubarb compote on top of the batter.
Add dollops of the custard, then swirl a skewer or cocktail stick through the compote to create a marbled effect.
Bake for 35-40 minutes until set and the edges are coming away from the sides of the tin.
Leave in the tin too cool.
Melt the remaining chocolate in a heatproof bowl set over a pan of simmering water.
Use a spoon to drizzle the melted chocolate over the top of the blondies in a zigzag pattern.
Cut into squares to serve. Will keep in an airtight container for up to 3 days.
12 servings