I would make one change though. Nigella used plain flour and bicarbonate of soda. It rose beautifully, but when cooled, it sank all over (nice and evenly thankfully). Next time I shall use S.R. Flour.
Here's the recipe as in Nigella's book for those of you that don't have *FEAST*
For the cake.
250g unsalted Butter.
400g Castor Sugar.
142ml Sour Cream.
1 tblsp Vanilla Extract.
275g Plain Flour.
2 1/2 teasp Bicarbonate of Soda.
For the topping.
300g Philadelphia Cream Cheese. 150g Icing Sugar. 125ml Double or Whipping Cream.
Preheat the oven to 180c, and butter and line a 23cm springform tin.Pour the Guinness into a large wide saucepan, add the butter, in slices and heat until the butter has melted, at which time you should whisk in the cocoa and sugar. Beat the sour cream with the eggs and vanilla, then pour into the brown, buttery, beery pan and finally whisk in the flour and bicarb. Pour the cake batter into the greased and lined tin and bake for 45 minutes to an hour. Leave to cool completely in the tin on a cooling rack, as it is quite a damp cake. When the cake is cold, sit it on a cake stand and get on with the icing. Lightly whip the cream cheese until smooth, sieve over the icing sugar, then beat them both together. Add the cream and beat again until it makes a spreadable consistency. Ice the top of the black cake so that it looks like the frothy top of the famous pint.