Preheat oven to 350°F Grease 24-30 cupcake cups, or fill with paper liners.
In a saucepan, heat the butter, Guinness, cocoa and brown sugar, whisking often, until the butter is melted and the mixture is smooth. Remove from heat and allow to cool to room temperature.
Into the work bowl of a stand mixer (or into a large mixing bowl), sift together salt, flour, white sugar and baking soda. Add the cooled Guinness mixture and beat on medium for 1 minute. Add eggs and sour cream and beat on medium for 2 minutes or until smooth.
Divide the batter evenly amongst the cupcake cups. Bake in preheated oven 20-25 minutes, or until a toothpick inserted into the center of a cake comes out clean. Cool in pan 20 minutes, then remove and transfer to a cooling rack until completely cooled.
Meanwhile, in (cleaned) work bowl of stand mixer, cream butter until very light and fluffy. Add salt, and slowly add confectioners sugar. Do not run the mixer above low, or the sugar will spray all over your kitchen. Add a couple of spoonsful at a time until it has all been absorbed into the butter. Add Bailey's and milk until spreadable consistency is achieved.
Frost cupcakes with Bailey's frosting. Sprinkle with green jimmies.