Preheat oven to 350 degrees F (175 degrees C). Line 12 muffin cups with paper liners.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Combine the flour, baking powder, baking soda, salt and cocoa; stir into the creamed mixture alternately with the milk.
Fill each liner about 2/3 full with batter. Bake for 20 minutes in the preheated oven, or until a toothpick inserted into the center of a cupcake comes out clean.