Hi Nigella Team, I've baked the Devil's Food Cake (Kitchen, p254) on two occassions now, using 20cm sandwich tins and following the recipe meticulously. The layers are very thin, only about 2cm each - what could be the reason for this? I have a Thermofan oven, so this also can't be the cause. Thanks!
The Devil's Food Cake uses both baking powder and bicarboante of soda (baking soda) as leavening agents. It is possible that one of these in your cupboard is out of date and so may not be working fully effectively. The "use by" date shoud be on the side of the packaging. It is also slightly possible that if "Dutch processed" cocoa was used (this is treated with alkalis, we have previously posted on the different type of cocoas), and as it is less acidic it would mean the bicarbonate of soda would be less active. However there is acid in the baking powder which will work with the bicarbonate of soda and we have made the cake with both types of cocoa without any problems, so we doubt that this is the cause.
We would also suggest that you check the Thermofan user's manual as the oven has several different functions. If using the Thermofan cooking function then you probably don't need to preheat the oven but it appears that you may have to use the special baking trays (and the base of the cake pans needs to be in full contact with the trays). The oven temperature also needs adjusting slightly. If using the conventional bake mode then you don't need the trays but you will probably need to preheat the oven and we would suspect that you need to use the temperature stated in the recipe.