Cream together the butter and confectioner's sugar in a mixer until fluffy. Add the cocoa powder, vanilla, and milk. Mix for an additional 3-4 minutes until smooth. If you want your frosting to be thinner and easier to spread, add a little more milk gradually.
Recipe by It's Always Someone's Birthday at http://www.itsalwayssomeonesbirthday.com/2011/12/angry-birds-cupcakes.html