This is a simple variation of the standard buttercream recipe.
Simply make a batch of buttercream (recipe can be found here.) Add 1/4 cup to 1/2 cup of cocoa powder and 2-4 tbsp. of milk (depending on desired consistency.) Beat until smooth. The measurements really do not need to be exact for this. When I make chocolate buttercream, I just start spooning in cocoa powder until it is as chocolatey as I want it.