I won't post a step-by-step on the cupcake recipe because really, I didn't make any changes to it and honestly, was a little disappointed in the results. In the future, I will try another chocolate cake recipe or just take the plunge and add some mayonnaise so that the cake is more moist. Here's the link and below, I've gone ahead and posted a slightly adapted frosting recipe.
UPDATE: I forgot to mention that the batter makes enough for 3 dozen cupcakes. I judiciously froze 2 dozen for another day when the chocolate craving hits. p.s. I am running out of room in our tiny freezer!
2 cups confectioners’ sugar
1 cup creamy peanut butter
5 tablespoons unsalted butter, room temperature
1 teaspoon pure vanilla extract
1/4 teaspoon salt
1/3 cup heavy cream
1/3 cup Irish Cream Liqueur
Place the confectioners’ sugar, peanut butter, butter, vanilla, and salt in the bowl of an electric mixer fitted with a paddle attachment. Mix on medium-low speed until creamy, scraping down the bowl with a rubber spatula as you work.
Add the heavy cream and Irish cream, beating on high speed until the mixture is light and smooth.
Continue to add confectioners’ sugar until desired consistency is reached (you might need more than the measurements - just keep an eye on it). Pipe or spread icing onto cooled cupcakes. Sprinkle on peanut butter cups as an optional garnish.
Note: Depending on how strong you want the flavour to come through in the frosting, the measurement for the Irish cream is relative to the baker. ;) Just make sure that the frosting consistency doesn't get too runny. If it does, just add more confectioners' sugar.