Beat butter in a small bowl with an electric mixer until creamy about 2 minutes.
Add sifted confectioners' sugar, cocoa powder, heavy cream, and vanilla extract. Beat on low speed for 30 seconds, gradually increase the speed and beat for another 30 seconds.
Notes
Add 3 tbsp more confectioners' sugar or cocoa powder if the frosting is too thin or another tablespoon of milk if the frosting is too thick.