Place the chopped rhubarb into a wide pan with caster sugar and 2 tbsp of water. Cook on a medium heat, stirring frequently, for about 10 minutes, or until the rhubarb breaks down and turns jammy. Set aside to cool.
Heat the oven to 180 C (160 C fan). Place the butter and both sugars in a medium pan over a low heat. Melt them together until smooth and glossy, remove from the heat and cool for about 10 minutes.
In a large bowl, sieve together the flour, custard powder and baking powder. Butter the baking tin and line it 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 white chocolate chips or chopped white chocolate and vanilla. Pour the batter into the baking tin then use a teaspoon to swirl the rhubarb on top of the batter.
Add dollops of custard, then swirl a skewer or a toothpick through the rhubarb and custard to create the marbled pattern. Bake for 35- 40 minutes until set and the edges are coming away from the sides of the tin. Leave in the tin to cool.
Melt the remaining 100g of white chocolate chips or white chocolate and drizzle it over the cooled blondies. Cut into equal squares and enjoy!
You can add few drops of pink food colouring to your rhubarb compote to intensify the colour
Instead of using fresh rhubarb, you can also use frozen or canned rhubarb (if using canned rhubarb, make sure to drain it first)
If you are not a fan of rhubarb, you can swap it for your favourite jam or compote