Guinness Chocolate Cupcake with Peanut Butter Fluff Frosting

Ingredients

• 12 ounces Guinness® stout
• 1/2 cup milk
• 1/2 cup vegetable oil
• 1 tablespoon vanilla extract
• 3 large eggs
• 3/4 cup sour cream
• 3/4 cup unsweetened cocoa powder, plus more for garnish
• 2 cups sugar
• 2 1/2 cups all-purpose flour
• 1 1/2 teaspoons baking soda
• 1 stick unsalted butter, softened
• 1 cup creamy peanut butter
• 4 cups confectioners’ sugar
• 1/2 cup marshmallow fluff
• 2-4 tablespoons milk