Whip butter and cocoa together until smooth in large bowl.
Stir in vanilla extract 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.
Frost your cake or cupcakes*!
Video
Notes
*Frosts one 9 or 8 inch round layered cake or about 24 cupcakes, depending on how much frosting you put on each one.
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.
Storage Instructions:This chocolate frosting will last for one week in the fridge as long as it's in an airtight container. If you plan on using it to frost a cake, you'll want to bring it to room temperature for 3-4 hours first and then whip it for a quick 20 seconds to get it light and fluffy again before using.