Servings: About 4 cups, enough for 1 (8-inch) four-layer cake

3 cups powdered sugar 3 cups powdered sugar
1/2 cup dark cocoa ½ cup dark cocoa
1 cup butter, room temperature 1 cup butter, room temperature
3 tablespoons Guinness 3 tablespoons Guinness

On a large piece of parchment paper, sift the powdered sugar and cocoa. Set side.
In a stand mixer fitted with a paddle attachment, beat the butter on high speed until very pale. Scrape down the sides of the bowl.
Turn the mixer speed to low and slowly add the powdered sugar and cocoa. Mix until well combined and then add the beer. Scrape down the sides of the bowl again.
Turn the mixer speed to high and let it run for at least 10 minutes, until the buttercream is light and fluffy.