In the bowl of a stand mixer fitted with whisk attachment, whisk butter and sugar on medium-high speed until very pale and fluffy or about five minutes. Be sure to stop at least once and scrape the bowl.
With the mixer off, add in cocoa powder, vanilla and salt.
Turn mixer on low and blend for about 30 seconds.
One tablespoon at a time add in heavy cream.
Once all cream has been added and mixture is mostly combined, turn off the mixer and scrape down the sides of the bowl.
Now turn the mixer onto medium-high to high and whisk for 3-5 minutes or until mixture is lighter and fluffier. The frosting will be shiny and seem to have many large air bubbles throughout and this is exactly what we are going for. (You can smooth those out on the cake if so desired.)