1. For cake: Preheat oven to 350°F. Butter two 9-inch round cake pans. Line with parchment paper. Butter paper.
2. Bring the stout and butter to simmer in heavy large saucepan over medium heat.
3. Add cocoa powder to the stout and butter mixture. Whisk until mixture is smooth. Cool slightly.
4. Whisk flour, sugar, baking soda, and 1 1/2 teaspoons salt in large bowl to blend.
5. 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.
6. Slowly Add flour mixture to the chocolate and egg mixture. Beat briefly on slow speed. Using rubber spatula, fold batter until completely combined. Divide batter equally among prepared pans. Bake cakes until tester inserted into center of cakes comes out clean, about 35 minutes.
7. Transfer cakes to rack; cool 10 minutes. Turn cakes out onto rack and cool completely.
8. For icing: Bring cream to simmer in heavy medium saucepan. Remove from heat. Add chopped chocolate and whisk until melted and smooth. Refrigerate until icing is spreadable, stirring frequently, about 2 hours.
9. Place 1 cake layer on plate. Spread icing over. Top with second cake layer. Spread remaining icing over top and sides of cake. Cut and enjoy.

Serves 12.