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.
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Notes
Can I use milk instead of cream? Yes, you can, but the texture will be softer and more mousse-like. It won't hold stiff peaks for piping details or crisp swoops on cupcakes, but it's perfect for filling and frosting a cake using a knife. See the question answered in blog post for a photo of cake being frosted with frosting made with milk instead of cream.
Beating the butter first separately starts to work in some air in before the cold cream firms it up and increases the surface area of the butter for the dry ingredients to be more easily incorporated.
Sift your confectioners' sugar and cocoa into the butter if you are going to pipe your frosting using fancy tips. This ensures no lumps, which could clog a small piping tip. I don't bother sifting if I'm just spreading the frosting on, as a 5 minute whip does a really thorough job of incorporating everything,
Make sure your cake is completely cool before you add frosting.
Do NOT use salted butter in this recipe. It will ruin your frosting. Unsalted butter is the only option.