Whisk together the butter and sugar on full speed until light and fluffy.
Add the eggs, then the milk and the vanilla extract.
Pour in the flour, then add the baking powder and the salt. Mix until you get homogenous batter.
Separate the batter into two smaller bowls. Add the cocoa powder to one and mix again.
Fill the cake mold with the help of two ladles (or normal tablespoons, depending on the pattern you want to obtain) altering the vanilla and chocolate batter.
When both batters are gone, take a fork, put it into the dough at one end of the mold and push it to the other end. Now you got the final swirl.
Bake for 50-60 minutes. Baking time depends on your oven. When a toothpick comes out clean, the cake is well baked.