Combine root beer and butter in a saucepan, once butter has melted, remove from heat. In a separate bowl, whisk together sour cream, eggs and vanilla. Add to cooled root beer and whisk until combined. Add in the cocoa and the sugar, mixing well. In a separate bowl, whisk together the flour and the baking soda. Add this to your root beer mixture, whisking until flour is incorporated. Batter will be very runny, but will bake up nicely. Fill cupcake liners about 3/4 full. I poured my batter into a large Pyrex measuring cup. This made pouring into my cupcake liners much easier. Bake in preheated oven for 15-17 minutes or until your cupcakes spring back when touched. Remove from oven and let cool completely before frosting.
For the buttercream
Combine softened butter and shortening in a large mixing bowl. Beat until very fluffy, about 10 minutes. Add in powdered sugar one cup at a time, mixing well after each addition. Add in the salt. Add in your bourbon cream. This is something you may have to taste as you go along (darn huh?). 4 tbsp to half of my buttercream added a subtle flavor. You may want to add more if you want the bourbon to be more pronounced. To make this kid friendly, I added about 4 tbsp of root beer to the other half of my buttercream. Root beer extract would be better, but the soda was just fine. Mix buttercream until creamy. Try to save some for the cupcakes!
* Irish Cream could be used instead of Bourbon Cream. Kahlua would be tasty too!
