I have been making the Chocolate Guinness Cake for years and came online whilst shopping to check ingredients and noticed that cornflour has been added to the icing. I am wondering why this has happened?
Nigella's Chocolate Guinness Cake (from FEAST) is a rich, moist cake that is topped with a light cream cheese frosting. The original recipe was written many years ago when cream cheese was often sold in blocks and was fairly firm when cold.
However cream cheese is now mostly sold in plastic tubs or containers and this cream cheese seems to be softer and spreadable straight from the fridge. Unfortunately sometimes when icing sugar (confectioners' sugar) is added to this type of cream cheese the sugar dissolves quickly and results in a very liquid frosting. Cornflour (cornstarch) helps to absorb excess liquid and keeps the frosting firm. Cornflour is sometimes added to icing sugar as an anti-caking agent so it is not unusual to have some cornflour in a frosting. However if you are able to buy a firmer style of cream cheese then it is not necessary to add the extra cornflour.