Now time to make the yummy chocolate buttercream frosting. First in a large bowl cream together your softened butter with your cocoa powder, using a mixer. Then add your vanilla extract, next add about 1 cup of your powdered sugar at a time, then mix, then add the sugar, etc. until it is all incorporated, scraping the sides of the bowl as needed. Now add about 1/3 of the milk, and see what kind of consistency you want. If you want the frosting thicker, leave it like this, if you want it a bit thinner, add another 1/3 of the milk. You may not need all of the milk. It just depends on how you want your frosting. :) All right our frosting is ready to go. If using a 9x13 pan you may want to reduce the frosting by half.