Preheat oven to 350°F.

Butter a 9-inch springform pan and line with parchment paper.

In a large saucepan, heat Guinness and butter over medium-low heat until butter melts.

Remove from heat, then whisk in cocoa and superfine sugar.

In a small bowl, mix sour cream, eggs and vanilla.

Add to the Guinness mixture.

Whisk in flour and baking soda until smooth.

Pour batter into the buttered pan and bake for 45 minutes to 1 hour, until risen and firm.

Let cool completely in the pan on a wire rack.

For the topping, blend confectioners' sugar in a food processor or by hand to remove lumps.

Add cream cheese and mix until smooth.

Gradually add heavy cream, blending until smooth and spreadable.

Remove cake from pan and place on a platter or cake stand.

Ice only the top of the cake to resemble the frothy head of a pint of Guinness.