Cupcakes:

Preheat oven to 350° Line 24 cupcake cups with liners. Bring the Guinness and butter to a simmer in a heavy, medium saucepan. Add cocoa powder and whisk until the mixture is smooth. Cool slightly.

Whisk the flour, sugar, baking soda and salt in a large bowl to combine. Using an electric mixer, beat the eggs and sour cream on medium until combined. Add the Guinness-chocolate mixture to the egg mixture and beat to combine. Reduce the speed to low, add the flour mixture and beat briefly. Using a rubber scraper, fold the batter until completely combined. Divide the batter among the cupcake liners. Bake until a thin knife inserted into the center comes out clean, about 17 minutes. Cool.

Whiskey Ganache Filling:

Finely chop the chocolate and transfer it to a heatproof bowl. Heat the cream until simmering and pour it over the chocolate. Let it sit for one minute and then, using a rubber scraper, stir it from the center outward until smooth. Add the butter and whiskey and stir until combined. Let the ganache cool until thick but still soft enough to be piped.

To fill the cupcakes, use a 1-inch round cookie cutter (or the bottom of a large decorating tip), cut the centers out of the cooled cupcakes, going about two-thirds of the way down. Transfer the ganache to a piping bag with a wide tip and fill the holes in each cupcake to the top.

Baileys Frosting

Beat the butter on medium-high speed for 5 minutes, scraping the sides of the bowl occasionally. Reduce the speed to medium-low and gradually add the powdered sugar until all of it is incorporated. Add the Baileys, increase the speed to medium-high and whip for another 2 to 3 minutes, until light and fluffy.

Frost the cupcakes using your favorite decorating tip and decorate with sprinkles. Store in an airtight container.