preheat oven to 375 degrees. beat the butter until creamy. add sugar and continue to beat until the mixture is light and fluffy. add eggs beating well. sift together the cocoa, flour, baking powder, and salt. add the dry ingredients and the milk to the egg mixture and beat until smooth. add the vanilla and beat well. transfer the batter to a greased and floured muffin tin, filling each 2/3 full. bake for 20 minutes. remove from tins and let cool on racks.