The most extraordinary moist Guinness Chocolate Cake made by preheating oven to 350°F greasing and lining 9×13 pan or two 8-inch round pans combining 1 cup Guinness Extra Stout and 1 cup cubed butter in heavy saucepan over medium heat until butter melted removing from heat whisking in 3/4 cup cocoa and 2 cups sugar until smooth cooling 10 to 15 minutes whisking in 3/4 cup room temperature sour cream 2 room temperature eggs and 1 tbsp vanilla whisking 2 cups flour 2 1/2 tsp baking soda and 1/2 tsp salt in large bowl pouring Guinness mixture into flour mixture folding just until combined pouring into pan baking 35 to 40 minutes until toothpick shows few moist crumbs cooling completely then beating 1 1/2 cups room temperature butter 3 minutes adding 4 to 5 cups sifted powdered sugar one cup at a time adding 3 to 4 tbsp Irish whiskey cream and salt beating medium-high 2 to 3 minutes until fluffy frosting cooled cake. A completely extraordinary moist Guinness stout Irish whiskey buttercream cake recipe.