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For the rhubarb & custard swirl, put the rhubarb and sugar in a wide pan with 2 tbsp water (omit the water if using canned rhubarb). Cook over a medium heat, stirring frequently for about 10 mins, until the rhubarb breaks down and turns jammy. Add a few drops of food colouring if you want a pink rhubarb swirl, but the flavour will still be great if you’re using green-tinged rhubarb. Leave to cool.
Heat the oven to 180C/160C fan/gas 4. Put the butter and both sugars into a pan and put over a low heat. Melt together until smooth and shiny, then remove from the heat, and leave to cool for 10 mins while you sieve the flour, custard powder and baking powder in a bowl. Butter a 20 x 30cm baking tin and line with baking parchment.
Beat the eggs into the cooled sugar and butter mixture, then fold in the dry ingredients until you have a smooth batter. Stir in 150g of the chocolate chips and the vanilla. Pour into the brownie tin, then use a teaspoon to swirl 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 pattern.
Bake for 35-40 mins until set and the edges are coming away from the sides of the tin, then leave in the tin to cool. Melt the remaining 100g chocolate in short blasts in the microwave or in a bowl set over a pan of simmering water. Use a spoon to drizzle the chocolate over the blondie in a zig-zag pattern. Cut into squares to serve. Will keep for three days in an airtight container, or freeze the squares individually.