Mix the cake mix, water, eggs and 1/3 cup of the butter in a medium sized bowl. Pour half of the batter in another medium sized bowl and mix in the rest of the butter, cocoa powder and black food coloring.
Preheat oven to 350 and line a muffin pan with cupcake liners. You should now have two colors of cake batter. Take 3 plastic piping bags that have not been cut yet. Fill one with the white batter and the other with the black batter. Place both bags in another bag evenly. Cut off the bag where the two tips meet see pic above. Now carefully fill the cupcake liners with the batters by squeezing the piping bags and holding them still above the liners.
Bake as directed on the box mix and let cool completely before icing or decorating!