Pre-heat oven to 350 degrees and line 2 cupcake tins with cupcake papers.
Place Guinness and cubed butter in a large saucepan over medium heat. Whisk until butter melts; remove from heat.
Whisk in brown sugar and cocoa powder.
In a separate small bowl, whisk together the sour cream, eggs, and vanilla. Whisk this mixture into the Guinness mixture.
In a separate large bowl, sift together flour, baking soda, and salt. Fold this mixture into the Guinness mixture. Take care not to over-mix; a few lumps in your batter is OK.
Bake cupcakes for 18-21 minutes, until a toothpick stuck into the center comes out clean. Let cupcakes cool in pan for a few minutes before moving to racks to cool completely.
Once cupcake are cooled, frost with Guinness Frosting. I use a Wilton #1A tip to frost mine. Top with sprinkles if desired.