Preheat oven to 350F
Line a 9 inch springform pan a with parchment paper, set aside.
In a saucepan over medium low heat, heat up the Guinness and Butter.
When the butter has melted, remove the pan from heat and whisk in cocoa powder and sugar.
In a separate bowl, combine sour cream, eggs and vanilla, then whisk this into cocoa mixture, until smooth.
Whisk in flour, baking soda and salt, whisk until everything is incorporated well.
Pour the batter into the prepared springform pan.
Bake 40-45 min or until cake is firm, and a toothpick inserted in the middle comes out clean.
Cool completely before icing.
While the cake is cooling, make the icing. In the bowl of a stand up mixer, beat together cream cheese and butter until smooth.
Gradually add sugar until just mixed, then beat in Bailey's until smooth.
Spread the icing only on top of the cake.