• Preheat oven to 350 degrees and line a 12-cavity cupcake tin with papers.
• Bring stout and  butter to a simmer in a large, heavy saucepan over medium heat. Add cocoa powder to the saucepan and whisk the mixture until it’s smooth. Remove saucepan from heat.
• In a separate medium bowl, whisk together the flour, sugar, baking soda, and salt.
• In the bowl of a stand mixer or in a separate large bowl with a hand mixer (or whisk) beat together egg and sour cream, until combined.
• Add the chocolate stout mixture to the egg mixture and beat until just combined.
• Add the dry mixture to the wet mixture and mix until just combined, taking care not to over-mix.
• Divide batter among cupcake liners, filling them about ¾ of the way.
• Bake for about 17-20 minutes, until a toothpick stuck into center of cupcake comes out clean.
• Let the cupcakes cool in the pan for a few minutes and then take them out to cool completely on a wire rack. Once cupcakes are cooled completely, core out a small section from the middle using either a knife or a cupcake corer.
• Spoon Irish whiskey filling into centers of cupcakes. Frost cupcakes with Baileys frosting. I used a Wilton 1A pastry tip for mine.

Irish Whiskey Filling

• Place the chocolate in a heatproof bowl. 
• In a small saucepan, bring cream just to a boil (keep a close eye on it and remove from heat right when it starts boiling). Pour cream over chocolate in bowl and let sit for 1 minute. Then, stir until chocolate is completely melted and smooth.
• Add butter, Baileys, and Irish whiskey, and stir to combine.

Baileys Frosting

• In the bowl of a mixer or in a large bowl with a hand mixer,  mix butter on medium speed until it’s nice and fluffy. Add confectioners’ sugar one cup at a time and beat until well-combined.
• Add the Baileys and beat until combined. If frosting is too thin, add more confectioners' sugar a couple tablespoons at a time.