Whisk in the flour, sugar, baking soda and salt in a large bowl to combine. Using an electric mixer, beat the eggs and sour cream on medium speed until combined. Add the Guinness-chocolate mixture to the egg mixture and beat just to combine. Reduce the speed to low, add the flour mixture and beat briefly. Using a rubber spatula, fold the batter together until completely combined. Divide the batter among the cupcake liners using an ice-cream scoop. I find that I can get a more equal amount by using this method. Bake until a toothpick inserted into the center comes out clean, about 17 minutes. Cool the cupcakes on a rack.