Preheat oven to 350.

Whisk together cocoa, sugar, flour, baking soda and salt.

In another bowl, combine the Guinness, melted butter and vanilla.

Beat in eggs and sour cream until combined and smooth.

Slowly add dry ingredients into the wet mixture.

Line muffin pan with paper liners.

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.

Add Bailey's until right consistency is achieved.

Transfer to piping bag and frost cupcakes.

Dust with a little cocoa powder if desired.