Preheat oven to 350 degrees and line cupcake liners with shamrock themed cupcake papers.
Heat the butter, Guinness, cocoa and sugar in a small sauce pan and whisk until the mixture is smooth. Set aside.
Medium mixing bowl, combine dry ingredients, flour, sugar, soda, salt, and the cooled Guinness mixer and mix on medium speed for a minute or until combined.
Add eggs and mix more until incorporated
Divide batter evenly into the liners and bake for 21 minutes.
Chocolate frosting ingredients
In a standing mixer, combine all ingredients and mix on low.
If mixture starts to look like rocks, add a tablespoon of heavy whipping cream.
Mix until stiff peaks form.
If too runny, add ½ C powder sugar.