For the dough, stir together the butter, sugar, vanilla sugar and salt until frothy. Mix in the eggs one at a time. Mix and stir in the flour, custard powder and baking powder.
Spread the dough in a greased springform pan (26 cm diameter). Spread the blackberries and apples on top.
Work the ingredients for the crumble with the dough or your hands into a crumbly dough and sprinkle evenly over the top.
Bake in the oven at 180 degrees for about 40-50 minutes.