Try this delicious cake recipe:
Mix custard powder, flour, baking powder, sugar and salt, knead with butter and eggs until smooth. Set aside to cool for 30 minutes.
Grease a cake springform pan of 22-24 cm ø. Line the pan with the rolled out dough, prick with a fork.
Bake the dough for 10 minutes at 185 °C. Select the blackberries and spread them evenly on the pre-baked dough.
Whip the egg whites until stiff, stir in the remaining ingredients. Fold in the egg whites. Spread the glaze evenly over the blackberries.
Bake at 175 °C for about 25 min (perhaps cover with aluminum foil after 10 min).