Preheat oven to 350.
In a large bowl, combine cocoa, sugar, flour, baking soda and salt. Whisk to blend.
In another bowl, combine the Guinness, melted butter and vanilla. Make sure all ingredients are at room temperature, to avoid curdling
Add in eggs and sour cream. Beat until combined and smooth.
Gradually add dry ingredients into the wet mixture.
Line muffin pan with paper liners or grease to avoid sticking
Fill each about 3/4 full.
Bake for 20-25 minutes until toothpick inserted in middle comes out clean.
Allow to cool in pan for 5 minutes before transferring to cooling rack.
Cream butter until fluffy.
Slowly add powdered sugar while beating until blended and proper consistency.
Add Bailey's and beat to incorporate.
Transfer to piping bag and frost cupcakes.