Step 1
For the cake: Heat oven to 350 degrees. Butter a 9-inch springform pan and line with parchment paper. In a large saucepan, combine Guinness and butter. Place over medium-low heat until butter melts, then remove from heat. Add cocoa and superfine sugar, and whisk to blend.
Step 2
In a small bowl, combine sour cream, eggs and vanilla; mix well. Add to Guinness mixture. Add flour and baking soda, and whisk again until smooth. Pour into buttered pan, and bake until risen and firm, 45 minutes to 1 hour. Place pan on a wire rack and cool completely in pan.
Step 3
For the topping: Using a food processor or by hand, mix confectioners' sugar to break up lumps. Add cream cheese and blend until smooth. Add heavy cream, and mix until smooth and spreadable.
Step 4
Remove cake from pan and place on a platter or cake stand. Ice top of cake only, so that it resembles a frothy pint of Guinness.