Sadly, the Chocolate Sheet Cake didn't turn out as well as I expected. Despite measuring everything properly, the cake rose in the middle and developed also a crack. Because of this, the icing pooled around the sides of the pan. The cake itself tasted under-sweetened and the icing is overly sweet. I am a huge fan of Nigella and love most of what she cooks. It's painful for me to write this.
Nigella's Chocolate Sheet Cake is a fairly large cake that is good for a crowd. It is also easy to make as it is a melt and stir method cake and doesn't require any special equipment. The cake will rise slightly more in the centre than at the edges, but should not rise excessively or develop a large crack in the centre.
If cakes rise a lot in the centre and crack on top it is usually due to one (or both) of two factors. Firstly if the oven is too hot then the edges cook quicky, pushing up the centre of the cake. If you have a fan oven then use the fan adjusted temperature or check your oven handbook, as ovens vary. If the cake pan is too small, this can also cause the centre of a cake to rise too much. The pan used is quite large and we would not go any smaller than a 30 x 20cm pan and if possible use the larger 33 x 23cm size. The frosting is very sweet and fudgy and the cake is less sweet to balance this. Usually the warm frosting partially soaks into the warm cake, balancing the two out. Unfortunately we understand that this may not have happened as much in this particular case if the cake had a large peak in the centre as the icing will run to the edges more.