Prepare all ingredients and bring them to room temperature.

Line a baking tin with parchment paper and grease it with butter.

In a mixing bowl, cream 200g butter, 170g sugar, and 1 tsp vanilla sugar until smooth.

Add 3 eggs one at a time and mix well.

Sift 250g flour and add it to the mixture along with 7g baking powder.

Add 2 tbsp milk and mix until combined.

Optional: Add 1 tbsp rum for flavor and mix gently.

Pour 2/3 of the batter into the prepared baking tin.

To the remaining batter, add 20g cocoa powder, 20g extra sugar, and 3 tbsp milk. Mix well.

Pour the chocolate batter over the plain batter in the tin.

Use a fork to create marble patterns by swirling the two batters together.

Preheat the oven to 180°C (356°F).

Bake the cake for 45–50 minutes at 180°C.