Preheat your oven to 350°F (175°C). Grease and flour a loaf pan.
In a large bowl, cream the butter and sugar together until light and fluffy.
Add the eggs one at a time, beating well after each addition.
In another bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the butter mixture, alternating with the milk. Stir in the vanilla extract.
Divide the batter in half. Stir the cocoa powder into one half.
Spoon the vanilla and chocolate batters alternately into the prepared loaf pan. Swirl with a knife to create a marble effect.
Bake for 60 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.