In a bowl of an electric mixer fitted with a whisk attachment, whip butter, cocoa powder, and salt over medium/high speed until combined. Turn the mixer off and add the confectioners' sugar all at once.

Turn the mixer on to the lowest speed and whip until the confectioners' sugar is completely combined with the butter/cocoa powder mixture. It will look super grainy/sandy at this point.

Stop and scrape the mixer to incorporate.

Turn the mixer to medium/high speed and add the heavy cream in 2 parts, allowing the heavy cream to thicken each time before adding in more cream. Add the vanilla extract and continue whipping the frosting until very light and fluffy, about 5 minutes.

Transfer to a container with a tight-fitting lid. Place in the refrigerator until ready to use.

Fills and frosts one 8-inch cake or one dozen cupcakes.