Preheat your oven to 350°F (175°C) and grease a loaf pan.
In a large bowl, cream the butter and sugar together until light and fluffy.
Add the eggs, one at a time, mixing well after each addition.
Stir in the vanilla extract.
In another bowl, combine the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the milk.
Divide the batter in half; stir cocoa powder into one half.
Spoon the batters alternately into the prepared pan.
Use a knife to swirl the batters together for a marbled effect.
Bake for 40-45 minutes or until a toothpick comes out clean.