The roasted barley in flavorful black stouts gives it a depth that makes it a favorite for baking. This moist cake is a delicious teatime treat; for dessert, serve it with berries and ice cream.
Preheat oven to 350F. Butter 8-inch round cake pan. Line with parchment or wax paper. Butter paper.
Bring stout and butter to a simmer in heavy saucepan over medium heat. Add cocoa; whisk until smooth. Cool slightly.
Whisk flour, sugar, baking soda and salt in large bowl to blend; set aside.
Beat egg, yolk and sour cream with electric mixer in another large bowl to blend. Add stout-chocolate mixture to egg mixture, and beat just to combine.
Add flour mixture and beat 30 seconds on slowest speed. Fold batter using rubber spatula until completely combined; pour into prepared pan.
Bake about 50 to 55 minutes, or until tester inserted into center of cake comes out clean. Transfer to rack; cool 10 minutes. Run small sharp knife around edge of cake to loosen and turn cake out onto rack; cool completely. Sift confectioners’ sugar over top, transfer to platter and serve.