Preheat the oven to 350° F. Butter your baking dish. Combine the Guinness and butter in a medium saucepan over medium heat. Add the cocoa powder and whisk till smooth. Remove from the heat and allow to cool slightly.
In a large mixing bowl, combine the flour, sugar, baking soda and salt. In another mixing bowl, beat together the eggs and sour cream to blend. Add the stout-butter mixture and beat just to combine. Mix in the dry ingredients on low speed just until incorporated. Spoon batter into buttered baking dish.
Bake until a toothpick inserted in the center comes out clean, about 25-35 minutes. Allow to cool in the pan for 5-10 minutes, then transfer to a wire rack to cool completely.
I served this with some homemade Irish Cream Liqueur. Éirinn go Brách!