I've been baking Nigella's Chocolate Fudge Cake for years, for my kids birthdays and events. We moved to the US a few years ago and something's going slightly wrong. There is a dark hard line near the base of each sponge layer. I use all-purpose instead of plain flour, superfine sugar instead of golden caster and light brown sugar instead of muscovado. I think everything else is the same. Is it the change in ingredients? Thanks!
Nigella's Chocolate Fudge Cake (from NIGELLA BITES) has two deep layers of rich chocolate sponge sandwiched with a chocolate fudge icing. There are a few possible causes of a hard layer in the cake. The US ingredients used sound correct and they should give a consistent sponge texture. However do check that the baking powder and bicarbonate of soda (in the US this is baking soda) are in date as if they are not then this could be the reason for a change in texture of the cake.
The cake layers are also fairly deep so there is a possibility that the line of hard mixture is being caused by uneven baking and could be that the oven is running cold or not maintaining an even heat, particularly if it is a standard gas or electric oven and not a fan oven. You could try checking the temperature of the oven with an oven thermometer and bake the cake on the centre shelf. Finally it could be that the cake batter is not thoroughly mixed. If you are using a freestanding mixer then after mixing give the mixture a couple of stirs with a spatula, making sure that you get right to the bottom of the bowl. The beater of a freestanding mixer sometimes doesn't quite get to the bottom of the bowl and leaves some batter that is not fully mixed. Also make sure that the eggs and sour cream are beaten together thoroughly before they are added to the mixture.