The richest, darkest, fudgiest frosting I've ever encountered.
Beat 1 1/2 cups unsalted butter on medium speed until light and creamy (about 5 minutes), scraping the bottom and sides of your bowl as you go.
Switch to low speed and add 1/2 cup powdered sugar, 1/2 cup cocoa powder, 1/3 cup coffee, 1/4 cup sour cream, 1 teaspoon vanilla and 1/8 teaspoon salt until combined (about a minute). Switch to medium speed and beat for another 2 minutes.
Add the melted chocolate and beat until smooth (about a minute).
Adapted from this recipe.