Preheat the oven to 350°F (175°C). Line two standard 12-cup muffin pans with paper cupcake liners and set aside.

In a medium mixing bowl, whisk together the flour, cocoa powder, espresso powder (if using), baking soda, baking powder, and salt until well combined. Set aside.

In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar together using an electric mixer on medium speed until light and fluffy, approximately 3 to 4 minutes.

Reduce the mixer speed to low. Add the eggs one at a time, mixing well after each addition. Add the vanilla extract and sour cream, and mix until the batter is smooth and well incorporated.

Gradually incorporate the dry ingredients into the wet mixture in three additions, alternating with the Guinness beer in two additions. Begin and end with the dry ingredients. Stir gently with a spatula or on low speed just until combined. Do not overmix.

Divide the batter evenly among the cupcake liners, filling each about ⅔ full. Bake for 16 to 18 minutes, or until a toothpick inserted into the center of a cupcake comes out clean or with a few moist crumbs. Allow cupcakes to cool in the pans for 5 minutes, then transfer to a wire rack to cool completely.

In a large mixing bowl, beat the butter on medium speed until smooth and fluffy. Gradually add 5 cups of powdered sugar, along with the vanilla extract, salt, and Baileys Irish Cream. Beat until the mixture is creamy. Add more powdered sugar, as needed, to achieve the desired consistency.

Once cupcakes are completely cooled, pipe or spread the frosting over each cupcake. Optionally, garnish with chocolate shavings, cocoa powder, or sprinkles. Serve and enjoy.