Whip butter and cocoa together until smooth in large bowl. Stir in vanilla and powdered sugar. Slowly stream in milk until frosting reaches desired consistency. Scrape sides and whip again until light, fluffy and smooth, 1-2 minutes. Frosts 1-9 or 8 inch round layered cake or 24 cupcakes.
Notes
If your frosting is too thick, slowly add in milk little by little to get the consistency you want.
If your frosting is too thin, add a little extra powdered sugar little by little to get the consistency you want.