Preheat oven to 350. Line two muffin pans with cupcake liners. Combine Guinness and butter in a medium saucepan over medium heat. When butter is melted, whisk in coca powder. Remove from heat and allow to cool slightly. In large bowl, combine remaining dry ingredients. In mixer bowl, blend eggs and sour cream. Add chocolate mixture and then dry ingredients, just until moist. Fill cupcake liners 2/3 full. Bake 17 minutes, until a toothpick comes out clean. Allow to cool 5 minutes before transferring to cooling rack. Cool completely before piping on the yummy frosting.