Knead cold butter in cubes, 40 g sugar, 125 g flour and 1 pinch of salt first with the dough hooks of the hand mixer, then with floured hands to form crumbles. Put the crumbles in a cool place.
Mix 175 g fat, 200 g sugar, 1 pinch of salt and vanillin sugar with the whisks of the hand mixer until creamy. Stir in the eggs one by one. Mix 275 g flour and baking powder. Mix flour and sour cream alternately into the dough.
Peel bananas and mash them with a fork. Loosely fold the bananas and frozen blueberries into the dough and place in a greased and floured box form (approx. 11 x 25 cm).
Spread crumbles on top and bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 50 minutes. Cover with foil about 10 minutes before the end of the baking time.
Remove the cake and let it cool on a cake rack. Remove the cake from the mould and dust with icing sugar before serving.