Preheat your oven to 350°F (175°C). Grease a loaf pan.
In a bowl, cream together the butter and sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition.
Mix in the vanilla extract.
In another bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the milk. Begin and end with the dry ingredients.
Divide the batter into two bowls. Stir the cocoa powder into one portion.
Spoon the vanilla and chocolate batters alternately into the loaf pan. Swirl with a knife for 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 transfer to a wire rack to cool completely.