Preheat the oven to 350°F. Grease a 9-inch springform pan with butter and line the bottom with a round of parchment paper. Dust with cocoa powder and tap out in excess.
Add the Guinness and butter to a large saucepan and heat over medium-high heat until the butter has melted. Whisk in the cocoa powder, sugars and espresso powder and mix until there are no lumps. Remove from heat and let come to room temperature.
In a medium bowl, beat together the sour cream eggs and vanilla extract until combined. Add to the stout mixture, whisking together.
In a medium bowl, whisk together the flour, baking soda and salt. Fold into the stout mixture until just combined.
Pour the batter into the prepared cake pan. Bake for 40-50 minutes or until a cake tester comes out clean.
Allow cake to cool before frosting.
Irish Cream Frosting
Using the whisk attachment of a mixer, whip the butter on medium-high speed for 5 minutes, scraping the sides of the bowl after every minute. Reduce the speed to medium-low and gradually add the powdered sugar until it is all incorporated. Add the Baileys, increase the speed to medium-high and whip for another 2-3 minutes until it is light and fluffy.
The Guinness cake can also be baked in an 8-inch or 10-inch springform pan but the baking time will vary.
All refrigerated items should be at room temperature, which helps the batter mix easily and evenly.
Use baking soda that is less than six months since it has been opened. If you use older baking soda, it won’t be powerful enough to help leaven the cake. You can test it by stirring half a teaspoon of baking soda into a cup of hot water. It will immediately start to fizz and release carbon dioxide gas if it’s still fresh enough to use.
You know cake is done when you insert a toothpick in the center and it comes out clean.
Allow the cake to completely cool before adding the frosting, otherwise it will melt.