Notes from the Test Kitchen:
What may at first seem to be an unlikely cupcake flavor turns out to be a real treat indeed! You can definitely taste all the different liquors in these sweet little cakes, so adjust the amounts to your liking.
Preheat oven to 350 degrees and line a muffin tin with cupcake liners. Combine the Guinness and butter (chopped into one-inch chunks) in a large saucepan and heat to melt butter.
Remove from heat and whisk in the cocoa and sugar.
In a bowl, whisk the sour cream, eggs, and vanilla, then add to the Guinness mixture.
Sift together the flour and baking soda, and fold into Guinness batter. Pour into muffin tins and bake for 25 minutes or until inserted toothpick comes out clean. Let stand for 10 minutes, remove from muffin tins and cool completely on a rack.
FOR GANACHE: Place the chocolate in a heat-safe bowl. Heat the cream in a small saucepan until simmering, then pour over the chocolate. Let sit for one minute then whisk until smooth. If the chocolate is not completely melted, place bowl over a double boiler. Add the butter and Bailey's and stir until combined. Set aside and cool until thick enough to be piped.
Meanwhile, cut out a portion from the center of each cupcake.
Pipe ganache into cupcakes.
FOR FROSTING: Put the stick of room-temperature butter in a large mixing bowl and beat at medium-to-high speed for a few minutes. Add a few short glugs (4-8 Tbsps) of Bailey's and and tiny bit of Jameson (to taste) and mix at a low speed. Gradually spoon the confectioner's sugar into the bowl, mixing on low speed. Frost the cupcakes as desired or use a piping bag to decorate.