Preheat the oven to 350 degrees F. Line two cupcake pans with liners.
Melt the butter in a saucepan over medium heat. Reduce heat to low. Add the beer, chopped unsweetened chocolate and cocoa powder and whisk until smooth. Remove from heat and set to the side and allow to cool.
In a large mixing bowl, combine the flour, cornflour, sugar, baking soda and salt, whisk until combined.
In the bowl of an electric mixer, beat together the eggs and sour cream until creamy. Slowly add the stout-butter mixture to the egg mixture, and mix just to combine.
Mix in the dry ingredients on low speed just until completely incorporated. Divide the batter evenly between the cupcake liners, filling them about 3/4 full.
Bake until a toothpick inserted in the center comes out clean (about 17 minutes). Allow to cool in the pan for 5-10 minutes, then transfer to a wire rack.