Preheat oven to 350 degrees F. Spray two 8-inch round cake pans with flour-added cooking spray.
Place butter in a saucepan and turn heat to medium. When butter is almost completely melted, add stout and cook over medium until mixture starts to simmer. Reduce heat slightly, add cocoa powder and stir or whisk until smooth. Remove from heat.
In a large bowl, thoroughly whisk together flour, sugar, baking soda and salt.
In a mixing bowl, using an electric mixer, beat eggs until thick and lemon colored – beat for a full five minutes. Add the sour cream and beat until combined. Add the stout mixture to the egg mixture and stir just until mixed. Add the flour mixture and stir until well mixed.
Divide the batter equally between the two pans. Bake for 28-32 minutes or until cakes test done (toothpick inserted comes out clean). Cool in pans for 10 minutes, then flip and let cool on a cooling rack.
To make the frosting, stir together softened butter, cocoa and 2 cups of the sugar. Add Guinness and vanilla and stir again. Using high speed of electric mixer (I use the whipping attachment on my stand mixer), beat icing for a minute. Add the rest of the sugar, adding just until you think it’s the right consistency. Beat for a good 3-5 minutes, scraping side of bowl often, until light and fluffy. Ice the cake.