Place butter and powdered sugar in a large bowl (or in the bowl of an electric stand mixer) When using the whisk attachment, whip on medium-high speed until pale and fluffy; roughly 5 minutes. Scrape the bowl at least once during this time.
Add the cocoa powder, vanilla, and salt, and blend on low for 1 minute. Add heavy cream in a slow drizzle as it mixes.
Once the mixture is mostly combined, turn off the mixer and scrape down the sides and bottom of the bowl well.
Whip buttercream on medium-high speed for 4-5 minutes, until it is impossibly shiny, light, and fluffy.
This recipe yields approximately 2.5 cups of frosting, enough to pipe on 16-18 cupcakes or fill and ice a 6-inch cake. Double the recipe if you need enough to decorate anything larger than a 6-inch cake fully.