Preheat oven to 300°F (150°C) and generously butter the loaf pan. You can also line the bottom with baking paper to prevent sticking.
Melt the butter and add the milk. Set aside to cool a bit.
In a large mixing bowl, beat the eggs and sugar with an electric mixer until very pale and fluffy.
Sift the flour, baking powder, salt, and vanilla sugar and fold into the egg mixture. If using vanilla extract, add it to the butter and milk instead.
Add the butter and milk mixture and fold until smooth.
Add the cocoa and milk to another mixing bowl and stir together into a smooth paste. Then add half of the cake batter and gently fold until the cocoa is incorporated.
Pour the chocolate and vanilla batter into the loaf pan in alternating layers, a bit at a time (I do two or three layers of each).
Bake for about 60 - 70 minutes, until the cake doesn't jiggle and a toothpick comes out clean.