1. Preheat your oven to 360 F.
2. Grease a medium sized bundt pan or any oven proof dish.
3. In a large bowl whisk the butter, sugar and vanilla till smooth. Then add the eggs, one at a time, and whisk until combined. Sift the flour, baking powder and salt together in a separate bowl. Add the sifted flour mix to the egg mixture in 2 batches, folding until the batter is smooth. Add the milk and fold until combined. Then transfer 1/3 of the batter into a separate bowl and fold in the cocoa powder until the batter is smooth.
4. Add the batter to your baking pan, alternating between the chocolate and vanilla. Tap the baking pan on your counter to get rid of any air bubbles, then swirl a toothpick or knife into the batter to make a marble design. Bake for 45 minutes or until a toothpick inserted into the center of the cake comes out clean. Cool the cake before slicing. Store in an airtight container.

This cake stays fresh for up to 3 days. Store it at room temperature in an airtight container to keep it from drying out.