Prepare cake mix according to cupcake directions on the box. Bake according to time on the box
Once the cupcakes have been cooked and cooled, prepare icing.
Melt butter in the pan or in the microwave. Add vanilla and cocoa powder. Stir until smooth. Add powdered sugar and milk. Add additional powdered sugar or milk for the desired consistency.
My icing was a little thick in the pictures. It was super fudgy when it set up but did not spread well and separated from the cupcake when you tried to eat it. Don't get me wrong, there is nothing wrong with having a big chunk of fudgy icing to eat straight but it is not the ideal outcome. If you look at the picture below, the icing has been thinned out with a little milk and was much easier to apply.