Preheat the oven to 350°.

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

Put Guinness and butter in a saucepan and melt together. Remove from heat. Add cocoa and superfine sugar and whisk. Mix together sour cream, eggs and Baileys and add to Guinness-butter mixture. Then gently whisk in flour and baking soda. Pour into pan and bake until risen and firm to touch—45 minutes to an hour. Cool on wire rack, then remove cake from pan.

For frosting, beat the cream cheese until smooth and sift in powdered sugar. Add mascarpone cream and mix until smooth. Frost just the top of the cake so it resembles a “frothy pint” of Guinness.