Preheat your oven to 350 degrees and line your cupcake pan with some kick ass green cupcake liners.
In a microwave safe bowl, heat up a little past melting, 1 stick of butter and 1 and 1/2 cup Guinness. Mix in 3/4 cup powdered, unsweetened cocoa and 2 cups dark brown sugar. I used Hershey's Special Dark Cocoa.
In a separate bowl, beat together 3/4 cup sour cream, 2 eggs, and 1 tsp vanilla. Whisk the butter and sour cream mixes together. Whisk in 2 and 1/2 cups AP flour and 1 tbsp baking soda into the wet mix.
Pour the batter into the the liners, filling 3/4th of the way full. Bake for 20 minutes. Check to make sure they're cooked through by inserting a toothpick into the thickest part of the thickest cupcake.
I deviated pretty seriously here, since I was just too lazy to go out and get some heavy whipping cream. So, this section is all me. Lol. In a microwave safe bowl, melt 3 tbsp butter, 3/4 cup semisweet morsels, and a bag (10.5 oz) of marshmallows. Pop it into the microwave and heat for 30 second intervals. Mix throughly after each 30 seconds.
Add Jameson to taste. I honestly couldn't taste it after 3 tbsp. So, I ended up using 5 tbsp because I only bought one of the 50ml bottles and ran out. I would have probably added more.
Hollow out the middle of the cupcake and pipe in the ganache. I used a plastic zip-top bag and cut a small hole in the bottom.
I went in a different direction with this too. The Bailey's is really strong in the frosting, so I only used 2 tbsp. The original recipe called for 4 to 8! That's crazy! Soften a stick of butter on the counter.
Use an electric mixer and beat the butter until light and fluffy. Add in 2 tbsp of Bailey's. Beat until combined.
Add 4 cups of confectioner's sugar, 1 cup at a time, so that the sugar doesn't go flying out of the bowl. Add in 2 tbsp milk, or until as fluffy as you'd like.
As with the ganache, put the frosting in a piping bag. I used a star tip. Decorate as you'd like. Try not to get drunk off cupcakes. Party hard, but please use a designated driver. I have few viewers, I'd like you all to remain alive. Kthxbai.