Ingredients
Cake
1 cup unsalted butter
1 cup Guinness stout
3/4 cup extra-dark unsweetened cocoa powder
2 large eggs
1 cup sour cream
1 tablespoon vanilla extract
2 cups all-purpose flour
1 cup granulated sugar
1 cup packed light brown sugar
1 1/2 teaspoons baking soda
3/4 teaspoon salt
Frosting
1/2 cup unsalted butter, at room temperature
4 ounces cream cheese, at room temperature
2 cups powdered sugar, divided
Instructions
Arrange a rack in the middle of the oven and heat to 350F/175C. Grease a 9-inch round cake pan that is 3 inches deep. You can also line the bottom of the cake pan with parchment paper for extra insurance if you’d like.
Melt the butter in a small saucepan over medium heat. Add the Guinness and bring to a simmer.
Remove the pan from the heat and whisk in the cocoa. Set aside to cool for at least 10 minutes.
Place the eggs, sour cream, and vanilla in a large bowl and whisk to to combine.
Add the cooled cocoa mixture to the egg mixture and whisk until smooth.
Add the flour, sugar, brown sugar, baking soda, and salt. Whisk until well-combined.
Pour the batter (it will be quite thin) into the prepared cake pan. The pan will be quite full.
Bake until the cake springs back when pressed in the middle or a skewer inserted in the center comes out clean, 50 to 55 minutes.
Cool the cake in the pan on a wire rack for 30 minutes. Remove the cake from the pan and cool on the wire rack for 30 minutes to 1 hour more before frosting.
Make the frosting:
Place the butter in a large bowl. Beat with electric hand mixer on medium speed until lightened in texture and smooth, about 1 minute.
Add the cream cheese and beat on medium speed until homogeneous and light, about 3 minutes.
Add 1/2 cup of the sugar, and beat until incorporated at medium speed. Repeat scraping and beating in the remaining powdered sugar in 3 to 4 more batches. Beat the frosting again on medium speed until smooth and slightly increased in volume, 3 to 5 minutes.
Chocolate Guinness cake should be frosted to look like a pint of stout with foam on top. Pour the frosting on top of the cooled cake and spread it just to the edge of the cake but not over the sides. Slice and serve immediately, if desired, or let sit at room temperature for up to 1 hour before serving.