1. Preheat the oven to 350 degrees Fahrenheit (177°C) and line a 12-cup muffin pan with cupcake liners.

To Make the Cupcakes:

1. In a small saucepan over medium heat, melt the butter with the Guinness and bring to a boil. Remove from the heat and stir in the cocoa powder. Allow the mixture to cool for about 15 minutes.
   
2. In a medium bowl whisk together the flour, granulated sugar, baking soda and salt. Set aside.
   
3. In the bowl of an electric mixer whisk the egg then, add the sour cream and stir on medium speed until combined.
   
4. Add about a third of the dry mixture into the sour cream mix, stir on low speed until almost combined, then add about 1/2 of the Guinness mixture. Add another 1/3 of the dry mixture, the remaining Guinness mixture and finish with the remaining dry mixture. Mix until just combined. Do not overbeat!
   
5. Pour or spoon the batter into the cupcake liners. Fill each about 2/3 full to avoid spilling or sinking and bake in the preheated oven for 20 minutes or until a toothpick inserted in the center comes out clean.
   
6. Remove from the oven and let the cupcakes cool in the pan for about 10 minutes then, place on cooling rack and cool completely before frosting.
   

Make the Chocolate Ganache:

1. Place the chocolate chips in a medium bowl. Set aside.
2. In a small microwave safe bowl or measuring cup, heat the cream until warm, about 15 to 30 seconds. You can also do this on the stovetop, in a small saucepan over medium-low heat. You want the cream to be warm without boiling (watch carefully!).
3. Pour the warm cream over the chocolate chips and allow to sit for 1 minute then, stir until smooth. Stir in the whiskey if using. Allow to cool a bit, until thick, but still pipable.
   
4. Place the chocolate ganache filling in a piping bag fitted with a round tip (0.25 or 0.5). You can also use a resealable bag with a corner cut off. Lastly a small spoon can do the job too!
5. With a spoon, small pairing knife, pipping tip or an apple corer, cut a small hole in the middle of the cooled cupcakes. Pipe some chocolate ganache in the center of each cupcake until it reaches the top.
   

To Make The Frosting

1. In a large bowl using a hand-held mixer or stand mixer fitted with a whisk attachment, whip the heavy cream until it starts to thicken about 2 minutes. Add the powdered sugar and Irish cream and whisk until just reaching stiff peaks, about 4 minutes.
   
2. Using a piping bag with your favorite decorative tip, pipe frosting onto each cupcake. (I used a star tip)