In the bowl of a mixer beat the butter until very pale in color and light. It should take about 6 – 8 minutes. Occasionally scrape down the sides of the bowl. 

Stir in the cocoa powder and then add the powdered sugar, salt, bourbon and the vanilla and 2 tablespoons of the cream.

Beat for 4 – 5 minutes more at high speed. 

You may need to add the final tablespoon of cream if it seems too thick. Conversely you can add in a small amount of extra powdered sugar if it seems too thin. Depending on air humidity or other factors.