Preheat oven to 350°F. Prepare an 18x13 cake pan by lining the bottom with parchment paper and spraying the entire pan with non-stick spray (you may also use butter).
Cut chocolate into small pieces for easy melting. Place chocolate, butter and stout in a metal or glass bowl and melt over a double-boiler until butter and chocolate are fully melted. Be careful not to scorch the chocolate.
In a large bowl sift together flour, cocoa, baking powder and salt.
In another bowl, whisk together the sugar and the eggs, just until they are combined.
Fold the melted chocolate mixture into the egg mixture.
Fold dry ingredients into the egg and chocolate mixture.
Fold in sour cream.
Bake for 35 minutes or until a skewer in the center of the brownie comes out clean.