Preheat oven to 350°F (175°C). Grease and flour a Bundt pan.
In a large bowl, cream together the butter and sugar until light and fluffy.
Add eggs one at a time, beating well after each addition.
Mix in the vanilla extract.
In another bowl, combine the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the milk.
Divide the batter into two bowls. Stir cocoa powder into one bowl.
Spoon the batters alternately into the prepared pan, and swirl with a knife to create a marbled effect.
Bake for 50 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.