Stir 100 ml milk, 75 g sugar and pudding powder until smooth. Boil 800 ml milk. Stir in the pudding powder and let it simmer for about 1 minute while stirring. Pour the pudding into a bowl, cover the surface with foil and let it cool down at room temperature for about 3 hours.
Finely mix 150 g biscuits in the universal chopper. Cream 300 g butter, vanillin sugar and 250 g sugar with the whisk of the hand mixer. Stir in the eggs one after the other. Mix flour and baking powder and stir alternately with 5 tbsp. milk into the dough.
Spread half of the dough on the greased fat pan of the oven (32 x 39 cm). Stir 2 tbsp. milk and biscuit crumbs under the other half, spread on the light-coloured dough. Sort the blackberries and spread them on the dough.
Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 20-25 minutes. Take out and let cool down. Whip 500 g soft butter with the whisk of the hand mixer for about 4 minutes until creamy.
Stir the pudding until smooth and stir into the butter by the spoonful. Pour the butter cream into a piping bag with a small star-shaped spout and squirt it onto the cake in small tuffs. Put the cake in a cool place for about 1 hour.
To decorate any 2 biscuits, remove the cream, crumble or chop the biscuits and sprinkle them on the cake. Cut the cake into pieces.