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.
in a small bowl, combine sour cream, eggs and vanilla; mix well. Add Guinness mixture. Add flour, salt and baking soda, and whisk again until smooth. Pour into buttered pan, and bake until risen and firm, 45 minutes to one hour. Place pan on a wire rack and cool completely in pan.
For the frosting: 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.
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.