Place 200 g flour, 30 g sugar and 1 egg in a mixing bowl. Melt 40 g butter. Warm the milk lukewarm. Dissolve yeast and 10 g sugar in the milk. Pour the milk mixture and butter into the mixing bowl and work everything with the dough hooks of the hand mixer, then knead it with your hands to a smooth dough. Lightly dust the dough with flour and let it rise covered in a warm place for about 30 minutes.
Meanwhile melt 110 g butter. Briefly mix 160 g flour, 90 g sugar, 1 packet vanilla sugar and the butter with the dough hooks of the hand mixer into crumbles. Put the crumbles in a cool place. Sort the blueberries. Stir the sour cream, 2 eggs, egg yolk, 35 g sugar, 1 packet vanillin sugar and starch until smooth. Knead the dough again on a floured work surface, roll out (approx. 30 cm Ø) and place in a baking tin (26 cm Ø) lined with baking paper. Sprinkle the base with breadcrumbs. Spread the blueberries on top. Pour the cast over it. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 40 minutes. After about 25 minutes, spread the crumbles on the cake and bake until done.
Knead the dough again on a floured work surface, roll out (approx. 30 cm Ø) and place in a baking tin (26 cm Ø) lined with baking paper. Sprinkle the base with breadcrumbs. Spread the blueberries on top. Pour the cast over it. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 40 minutes. After about 25 minutes, spread the crumbles on the cake and bake until done. Remove the cake from the oven and dust with icing sugar just before serving. Serve hot or cold