1. Preheat oven to 175°C | 350°F. Grease a 20cm (8 inch) round cake pan with butter. Melt butter in a saucepan or in the microwave. Set aside.
2. In a large bowl, mix butter with sugar until just combined.
3. Add the eggs, one at a time, beating after each addition. 
4. Add the yoghurt and mix well.
5. Divide the batter into two bowls.

For the Vanilla Batter

1. Add the 1/2 tsp vanilla extract, 150g sifted flour and 1/2 tsp baking powder, mix until incorporated.

For the Chocolate Batter

1. Add 100g sifted flour, 20g cocoa powder and 1/2 tsp baking powder, mix until incorporated.
2. (The batter will be smooth, velvety, and slightly thick. If the batter is too thick, you can add the milk. Or if the batter is too liquid, you can add the flour.)
3. Drop a large spoonful of 1 batter in the center of the pan. Top with a spoonful of the other batter. Repeat this steps until all batter has been used.
4. Bake in 350°F (175°C) for 40 minutes, until inserted toothpick comes out clean.