Bring the milk to the boil and stir in the powder. Let the milk cool down. Mix fat, sugar, vanillin sugar and salt in a bowl until creamy with the whisks of the hand mixer. Stir the eggs one after the other into the fat-sugar mixture.
Also stir in the milk. Mix flour and baking powder and stir into the mixture quickly. Roughly chop the chocolate and fold in.
Grease a fat pan of the oven (36 x 41 cm) and dust with flour. Add the dough and smooth it down. Bake the cake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 40-45 minutes.
Spread blueberries on the cake after about 15 minutes of baking time. Remove the cake and let it cool on a cake rack. Dust with icing sugar.