Place softened butter and 1 cup powdered sugar in a large bowl (or in the bowl of an electric stand mixer) Using the whisk attachment, mix until combined, scrape the bowl, and repeat with remaining sugar, adding it 1 cup at a time. When all sugar has been added, turn speed to medium-high and whip about 5 minutes, until pale and fluffy, scraping the bowl as needed.
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.
Whip buttercream on medium-high speed for 4-5 minutes, until it is silky, light, and fluffy.
