Add butter to bowl of stand mixer fitted with the whisk attachment. Start on low speed then gradually increase to high and beat for about a minute until butter is soft and fluffy.
Add confectioners' sugar, cocoa powder, cream, vanilla and salt. Beat on low speed until the dry ingredients are incorporated, then increase speed to high and whip the frosting for a full 5 minutes, pausing a couple of times to scrape down sides and bottom of the bowl.
