chocolate whiskey and beer cupcakes

INGREDIENTS

• 1 cup stout (such as 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
• (Updated to double it, based on many commenters suggestions — thanks!)
• 8 ounces bittersweet chocolate
• 2/3 cup heavy cream
• 2 tablespoons butter, room temperature
• 1 to 2 teaspoons Irish whiskey (optional)
• (see Recipe Notes)
• 3 to 4 cups confections sugar
• 1 stick (1/2 cup or 4 ounces) unsalted butter, at room temperatue
• 3 to 4 tablespoons Baileys (or milk, or heavy cream, or a combination thereof)