Preheat your oven to 160C / 320F and adjust a rack to the middle shelf.
Grease a 9x9 inch square tin with butter and line with baking parchment.
In a medium bowl, sift the flour, custard powder and baking powder then mix to combine.
In a medium saucepan, melt the butter and sugar over a low heat with a pinch of salt, then remove from the heat and leave to cool completely.
Once the mixture has cooled, transfer it into a large bowl and whisk in your egg.
Add the flour mixture, stirring until just combined then add the chopped chocolate mixing until everything is nicely incorporated.
Using a rubber spatula, swirl through the jammy stewed rhubarb and vanilla custard, leaving a little to dollop over the top.
Transfer the batter into the prepared tin and smooth into an even layer. If you're left with a little rhubarb mixture and custard, dollop onto the top of the blondie mix and use a knife to swirl.
Bake for 25-30 minutes until golden and bubbling. Partille N.B. The rhubarb will have thickened slightly, and the custard will brown a little. While it may look a little wet in the centre, the blondies will continue to cook once removed from the oven.
Remove from the oven and leave to cool in the tin for at least 15 minutes.
Lift the blondies out of the tin and transfer to a wire rack to cool completely then slice and enjoy!