In a large bowl, cream butter on medium speed until smooth.
Add confectioner’s sugar, cocoa, vanilla and heavy cream/milk. Beat until light and fluffy. Turn mixer off. Scrape down sides of bowl. Mix for an additional 1 minute.
If frosting is too thick, add milk, 1/4 teaspoon at a time. If the frosting is too runny, add confectioners sugar until consistency desired is reached.
Frost cupcakes as desired.