Preheat the oven to 350° F. Butter a 9″ springform pan and line the bottom with a disc of parchment paper cut to size.
In a large saucepan, simmer the stout and butter over medium heat. Add cocoa powder and sugar and whisk until smooth. Allow to cool.
In a large bowl, sift together flour and baking soda. In another large bowl, beat eggs, vanilla, and sour cream, then and add to the cooled stout mixture. Mix until thoroughly combined. Add the flour mixture and fold the batter until completely combined.
Pour the batter into the prepared pan and bake for about 50 minutes, until a toothpick inserted into the center of the cake comes out clean.
Cool completely on a wire rack. Then remove the cake from pan and transfer to a serving platter or cake plate. Frost and serve.
