Heat the oven to 180/160 fan/gas 4. Butter and line the bottom of a 23cm cake loose-bottomed cake tin.

Pour the Guinness into a saucepan and add the cubed butter. Bring to a gentle simmer, then turn down the heat and stir until the butter has melted. Remove from the heat.

Whisk in the cocoa powder and sugar until well combined. Beat the eggs in a jug with the soured cream and vanilla paste, then pour into the Guinness mix.

Finally, stir in the flour and bicarb. Pour the cake mix into the tin and bake for 50-60 mins. A skewer inserted into the middle of the cake should come out clean. If not put back in the oven and check again after 10 mins. Leave to cool in the tin.

Meanwhile, make the topping. Beat the soft cheese until soft, then beat in the icing sugar a spoonful at a time along with the cornflour. Whip the cream in a separate bowl to soft peaks, then stir into the cheese mix, one spoonful at a time. Spread the icing all over the cake, then slice to serve. Will keep chilled for up to a week.