Add the butter and powdered sugar to the bowl of a stand mixer (or use a hand held mixer). Beat the butter and powdered sugar together until the butter is light and fluffy, about 4 minutes. Add the cocoa powder and vanilla and beat, scrapping down the sides as needed another 2 minutes or until there are no streaks of white. Add 3 tablespoons of the heavy cream and whip the frosting for 2-4 minutes or until light and fluffy. If desired add the remaining tablespoon of the heavy cream (I normally do) and whip until combined. Taste the frosting and add more powdered sugar if you like a sweeter or thicker frosting.