Mix the butter, flour, sugar, eggs, baking powder and vanilla sugar together and spread the dough onto a baking sheet lined with baking paper. Bake at 180 ° C top and bottom heat for 15 minutes.
Prepare the milk and custard powder according to the instructions on the package (with sugar, see package). Let cool a little, but not too long, otherwise there will be lumps in the mass. Stir in sour cream and egg yolk and spread on the bottom.
Drain the sour cherries, raspberries and blueberries well (catch the juice!), Distribute over the mixture and bake for another 25 minutes.
Make the icing from the collected juice and cake icing according to the instructions on the packet and distribute it on the cake.