Preheat oven to 350°F.

Butter a 9-inch cake pan, line the bottom with baking parchment, and butter parchment.

In a large saucepan, heat Guinness and butter until butter is melted.

Whisk in sugar and cocoa.

Whisk in sour cream, vanilla, and eggs.

Whisk in flour, baking soda, and baking powder.

Pour into prepared pan, and bake 50 to 60 minutes, or until set and top is firm.

Cool completely in pan on a rack, 2-3 hours.

Run a dull knife around the edge, invert on a plate, remove parchment, and turn right-side up.