Modified slightly from Epicurious.com (from Bon Appetit-a recipe requested from the Barrington Brewery in Great Barrington, Massachusetts).
2 cups stout (I used Guinness-almost 2 bottles, drink the leftovers)
2 cups (4 sticks) unsalted butter
1 1/2 cups Dutch-process cocoa powder
4 cups all purpose flour
4 cups sugar
1 tablespoon baking soda
1 1/2 teaspoons salt
4 large eggs
1 1/3 cups nonfat yogurt (what I used) or sour cream
2 cups whipping cream
1 pound semisweet chocolate, chopped
Preheat oven to 350°F. Butter three 8-inch round cake pans with 2-inch-high sides, bundt pan, or put paper liners in cupcake tins (can make cupcakes in batches). Butter very well-ideally using the Baker's Joy spray.
Bring 2 cups stout and 2 cups 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 1 1/2 teaspoons salt in large bowl to blend. Using electric mixer, beat eggs and yogurt 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. If making full recipe, you'll probably have to add the last of the flour by hand, even if using a Kitchenaid, as it makes so much batter. 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 (Start cupcakes for 15 minutes; probably will take longer.) Transfer cakes to rack; cool 10 minutes. Turn cakes out onto rack and cool completely.
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. If you refrigerate too long and it gets too hard, let it sit on the counter or zap briefly on low in the microwave to soften enough to spread.
If making bundt cake, spoon ganache over cake while still fairly warm.
If making layer cake, place 1 cake layer on plate. Spread 2/3 cup icing over. Top with second cake layer. Spread 2/3 cup icing over. Top with third cake layer. Spread remaining icing over top and sides of cake.