Prepare the batter: In a mixing bowl, beat the softened butter, sugar, and vanilla sugar together until smooth and creamy. Add the eggs one at a time, continuing to beat for a consistent texture.

Combine dry and wet ingredients: Mix the flour and baking powder into the liquid mixture alternately with 1/3 of the milk. Save the remaining milk for later use.

Prepare the loaf tin: Grease a 30 cm long loaf tin and dust it lightly with flour. Pour half of the prepared batter into the tin.

Create the chocolate batter: Beat the remaining half of the batter with cocoa powder and the rest of the milk until smooth.

Layer the batters: Make a lengthwise hollow in the light batter using a tablespoon and pour the chocolate batter into it. Smooth out the surface for an even top.

Bake: Preheat the oven to 180°C (top and bottom heat) and bake the cake for 45–50 minutes. Test with a toothpick or skewer—if it comes out clean, the cake is ready. Avoid overbaking to keep the cake moist.

Cool and serve: Allow the Topfkuchen to cool in the tin before turning it out. Slice and serve as desired.