Preheat oven to 350 degrees F(set oven rack to second-lowest position).
Grease three 9 or 10-inch round cake pans then line the bottoms with parchment paper, then butter the paper.
Bring 2 cups stout beer and 2 cups butter to a simmer in a large saucepan over medium heat.
Add in cocoa powder and whisk until mixture is smooth; cool slightly.
In a large bowl whisk the flour, sugar, baking soda and 1-1/2 teaspoons salt.
In another bowl using an electric mixer beat eggs until thick and lemon coloured (about 5-6 minutes, this is important must beat for no less than 5 minutes!) add in the sour cream and beat until well combined.
Add the stout/chocolate mixture to the egg mixture; beat JUST to combine.
Add in the flour mixture and beat on low speed until combined.
Divide the batter equally among the three pans.
Bake for about 35 minutes, or until the cakes test done.
Cool in pans for 10 minutes, then remove to plates.