Preheat the oven to 350 F and prepare a 24 count cupcake pan with liners.
In a large sauce pan over medium, heat Guinness and butter until the butter melts and begins to lightly simmer. Add the cocoa powder and whisk until smooth. Set aside and let cool while working on the next step.
In a mixing bowl, combine flour, sugar, baking soda, and salt. Lightly whisk to combine.
In a larger mixing bowl, or the bowl of a stand mixer, combine eggs, sour cream, and coffee. Beat eggs until blended. Add the slightly cooled beer mixture to the eggs and beat until just combined. With the speed on low, add flour mixture, adding it about 1/3 to 1/2 at a time. Beat until just combined then pour mixture into the cupcake pan, filling the liners about 2/3 to 3/4 full.
Bake until they pass the toothpick test, about 17 minutes. Remove from pan and let cool on rack until completely cooled, about 1 hour.