Preheat oven to 350°F.
Line 24 cupcake cups with liners.
Bring 1 cup stout and 1 cup butter to simmer in heavy large saucepan over medium heat.
Add cocoa powder and whisk until mixture is smooth. Cool slightly.
Whisk flour, sugar, baking soda, and 3/4 teaspoon salt in large bowl to blend.
Using electric mixer, beat eggs and sour cream in another large bowl to blend.
Add stout-chocolate mixture to egg mixture and beat just to combine.
Add flour mixture and beat briefly on slow speed.
Using rubber spatula, fold batter until completely combined.
Divide batter among cupcake liners, filling them 2/3 of the way.
Bake cupcakes until tester inserted into centers comes out clean.
Cool cupcakes on a rack completely.