For the Guinness whoopie pies:
1 cup Guinness stout
16 tbsp. unsalted butter
¾ cup unsweetened cocoa powder
2 1/2 C. all-purpose flour
2 1/4 C. sugar
1½ tsp. baking soda
1 tsp. baking powder
¾ tsp. salt
2 large eggs
2/3 C. sour cream

For the filling:
16 T. (2 sticks) unsalted butter, at room temperature
4 cups confectioners’ sugar, sifted
6-8 T. Bailey’s Irish cream