Preheat the oven to 350°F (175°C).

In a mixing bowl, cream together the butter and sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

Stir 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.

Divide the batter in half and mix cocoa powder into one half.

Spoon the batters into the muffin tin, alternating between the vanilla and chocolate batters.

Bake for 20-25 minutes or until a toothpick comes out clean.

Let cool before serving.