Preheat oven to 350º.
Butter a 9-inch springform pan and line with parchment paper.
In a saucepan, combine Guinness and butter.
Place over medium heat until butter melts, then remove from heat.
Add cocoa and sugar.
Whisk to blend.
In a mixer, add sour cream, eggs and vanilla.
Beat to combine.
Add Guinness.
Add flour and baking soda.
Mix smooth.
Pour in pan and bake approx 35 mins or until toothpick comes out almost clean.