Preheat oven, prepare pan: Preheat the oven to 350°F. Grease a 8 or 9-inch springform pan with butter and line the bottom with a round of parchment paper.
Heat Guinness, butter, add cocoa and sugar: In a large saucepan place the stout and butter. Cook over medium-high heat until the butter has melted.
Add the cocoa powder and sugars and whisk together. Take off heat and allow to come to room temperature.
Beat sour cream, eggs, vanilla, then add to stout butter mixture: In another bowl beat together the sour cream, eggs, and vanilla extract until very, very well combined.
Add to the butter-stout mixture and whisk together.
Whisk flour, baking soda, salt, add to stout-butter mixture: In another bowl whisk together the flour, baking soda, and salt.
Add to the stout-butter mixture and whisk together until it just comes together.
Bake:
Pour into the prepared pan and give the pan a few short drops onto the countertop to shake the air pockets in the batter to the surface and out of the cake.
Bake at 350°F for 50-60 minutes.
Cool: Allow to cool on a wire rack. Run a knife around the edge to separate the cake from the pan and pop the ring off the springform.