Beat the butter until smooth, stir in the sugar and vanilla sugar. Add one egg at a time and stir well. Mix flour and baking powder and stir in briefly. Put the dough in a greased springform pan.
Mix the custard powder, milk, sugar and egg yolk until smooth, bring to the boil. Beat the egg white until stiff and fold in, leave to cool.
Wash the blueberries and add them to the cooled pudding, stir. Spread the mixture on the dough.
For the crumble, work through all the ingredients with your hands. Spread on the cake. Bake at 180 ° C on the middle rack for about 60 minutes.