Preheat your oven to 350°F and add 16 cupcake liners to a muffin tin and set aside.
In the bowl of a stand mixer, equipped with a whisk attachment, add the flour, sugar, cocoa powder, baking powder, baking soda, espresso powder and salt. Whisk until all ingredients are combined.
Slowly pour in the milk, Guinness, vegetable oil, eggs, and vanilla extract into the dry ingredients and mix until there are no dry spots. Do not overmix.
Evenly divide the batter between 16 cupcake liners and bake at 350°F for 15-16 minutes or until a toothpick inserted in the center of the cupcakes comes out clean.
Allow to cool for 5 minutes in the pan then transfer to a wire rack to cool completely.
Bailey’s Frosting:
Add the softened butter into the bowl of a stand mixer equipped with a paddle attachment and cream for 2-3 minutes until smooth.
Scrape the sides and bottom of the bowl. Pour in vanilla extract and mix well.
Add the powdered sugar one cup at a time. Fully combine after each addition. Scrape the sides and bottom of the bowl and mix until well combined.
With your mixer on low speed. slowly pour in the Bailey’s Cream. Once combined, increase speed to high for 2-3 minutes.
Add frosting to a piping bag equipped with a large star tip (I used a Wilton 1M).
Pipe a generous swirl of frosting on top of each cooled cupcake.
Add sprinkles on top of each cupcake, if desired, and serve.
Keep cupcakes in an airtight container at room temperature for up to three days.