Preheat oven to 350Â° and grease and line with parchment paper a 9-inch springform pan.
Pour the Guinness into a large saucepan and add the sliced butter.
Heat until the butter is melted and remove from the heat.
Whisk in the cocoa and sugar. In a separate bowl, beat the sour cream with the eggs and vanilla. Add the sour cream mixture to the Guinness mixture in the saucepan. Finally, beat in the flour and baking soda.
Pour the batter into the greased and lined springform pan and bake for 45 minutes to an hour. (I baked mine for 53-55 minutes, checking with a toothpick for doneness and it came out well. Your oven may vary.) Cool completely in the pan on a wire rack.
For the frosting, beat the powdered sugar and cream cheese together until well combined and creamy. Add the cream and beat again until it’s a spreadable consistency. I found slowly adding the cream and beating the icing very well gave excellent results.