Melt butter for the crumbles. Mix flour, 200 g sugar and baking powder in a bowl. Add egg and butter. First use the dough hooks of the hand mixer, then work into crumbles with your hands. Put about half of it on the bottom of a greased square springform pan (24 x 24 cm), press it to a flat bottom, chill it.
Stir pudding powder, 3 tablespoons sugar and 6 tablespoons milk until smooth. Bring the remaining milk to the boil, stir in the pudding powder, bring to the boil and simmer for about 1 minute. Remove from the stove. Place the foil directly on the surface and let it cool down for about 30 minutes. Sort blueberries, wash and pat dry. Mix blueberries and starch together. Spread the pudding on the crumble base and smooth it down, first sprinkle blueberries and then the rest of the crumble over it.
Bake in the preheated oven (electric oven: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) on the lower rack for 60-70 minutes. Let the cake cool down with the oven door open. Remove from the tin and cut into pieces.