For the Cupcakes:
1 cup Guinness stout
1 cup unsalted butter, at room temperature
¾ cup Dutch-process cocoa powder
2 cups all-purpose flour
2 cups granulated sugar
1½ teaspoons baking soda
¾ teaspoons salt
2 eggs
2/3 cup sour cream

For the Whiskey Ganache Filling:
8 ounces bittersweet chocolate
2/3 cup heavy cream
2 tablespoons butter, at room temperature
2 teaspoons Irish whiskey

For the Baileys Frosting:
2 cups unsalted butter, at room temperature
5 cups powdered sugar
6 tablespoons Bailey's Irish Cream