• Preheat the oven at 350 degrees and line a cupcake tin with liners.
• In a saucepan over medium heat, bring the Guinness stout beer and butter to a simmer.
• Add the cocoa powder and wisk until smooth, then remove from heat.
• In a large bowl, combine your dry ingredients and set aside.
• In a stand mixer, beat the eggs and sour cream until combined.
• Add the Guinness-chocolate mixture and beat until well combined.
• Add the flour mixture and beat until mixed well.
• Fill each cupcake liner about 2/3 of the way and bake for 17 minutes.

Frosting

• Beat the butter and powdered sugar until well combined.
• Add the vanilla and Bailey's and mix well.
• Slowly add the milk 1 tablespoon at a time until the frosting is fluffy and smooth.
• Fill a piping bag or plastic storage bag with icing and pipe onto the cupcake.