For the rhubarb mix all the ingredients together in a tray and roast for 30 minutes at 180C. Once cooked drain the juice off and use as a cordial or for a Rhubarb Floradora. Preheat the oven to 180°C/160°C fan and line a 23cm square tin with parchment paper.
Cream the butter and sugar together until smooth. Add the eggs and beat until smooth, then add the plain flour and custard powder and beat until combined. Fold through the white chocolate chips or chunks. Pour the mixture into the tin and spread out evenly.
Dollop the drained rhubarb mixture and custard over the top and swirl through slightly using a cake skewer or knife.
Bake for 35-40 , or until there is an ever so slight wobble in the middle. Leave the blondies to cool fully in the tin before cutting into 16 squares.