For the Guinness Chocolate Cupcakes

1 cup Guinness
1 cup (2 sticks) unsalted butter
3/4 cup unsweetened cocoa powder (preferably Dutch-process)
2 cups all purpose flour
2 cups sugar
1 1/2 teaspoons baking soda
3/4 teaspoon salt
2 large eggs
2/3 cup sour cream (I used light)

Baileys Irish Buttercream
3 to 4 cups confections sugar (I used about 3.5 cups)
1 stick (1/2 cup or 4 ounces) unsalted butter, at room temperature
3 to 4 tablespoons Baileys Irish Cream (I used 3 tbsp Bailey's and 1 tbsp milk to get it to the right consistency)