Mix butter, sugar and vanilla sugar with the whisk of the hand mixer for about 5 minutes until creamy. Mix flour, cocoa and baking powder. Stir in the eggs one after the other. Alternately stir the flour mixture with the milk into the egg fat cream. Pour the dough into a greased, flour-spread box form (11 x 30 cm), smooth it down. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 45-55 minutes (stick test).
Take the finished cake out of the oven and let it cool down for about 15 minutes. Then carefully remove from the mould and let it cool down.