Do you need to use the cornstarch (cornflour) in the icing for the Chocolate Guinness Cake if you’re using the brick cream cheese in the US?
Nigella's Chocolate Guinness Cake (from FEAST) is topped with a swirl of cream cheese frosting that emulates the frothy "head" on a pint of Guinness. The frosting includes some cornflour (cornstarch) as this helps to prevent the frosting from becoming too runny if the more spreadable tub form of cream cheese is used.
Originally the recipe used a firmer form of cream cheese (similar to the "brick" type still sold in the US), so in theory you should not need the cornflour. But some brands of icing (confectioners') sugar used to have cornflour added as an anti-caking agent, which helped to absorb some of the liquid and keep the icing firm, and are now using other agents instead. If you find that the frosting is becoming too liquid when you are adding the icing sugar then stir in the cornflour and let the frosting stand in a cool place for about 15 minutes (to let the cornflour absorb some of the liquid) before continuing.