Preheat your oven to 350°F (175°C). Grease and flour an 8-inch cake pan.
In a mixing bowl, cream together the butter and sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
In another bowl, combine the flour, baking powder, and salt. Gradually add this to the butter mixture, alternating with the milk, beginning and ending with the flour mixture.
Divide the batter into two bowls. Stir the cocoa powder into one portion of the batter.
Spoon the batters alternately into the prepared pan, and swirl with a knife to create a marbled effect.
Bake for 45 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.