In a medium mixing bowl, cream butter to lighten and aerate it slightly. Sift powdered sugar and cocoa powder over the butter. Mix everything together until you get a dry, crumbly mixture.
Add in your milk or cream and vanilla. Mix until the dry ingredients are fully moistened and you have a dense, liquid mixture. ** see notes
Using a mixer fitted with a paddle attachment, whip the frosting on medium high power for about 5 minutes, or until it turns a light brown color and it almost doubles in volume.