I just made the Christmas Spiced Chocolate Cake and followed the recipe exactly as I always do the first time I try something. The cake not only "fell", it cracked, sunk and broke at one side. Did I make an error? Baked too long, not long enough? After 35 minutes the top was firm and I took it out, left in cake pan on rack covered with tea towel. I have some choc orange slices which I will stick in before I serve to make it look deliberate!
Nigella's Christmas-Spiced Chocolate Cake (from NIGELLA CHRISTMAS) is a type of cake known as a "fallen" chocolate cake. It does not contain any chemical leavening, only the air whisked into the eggs. These type of cakes sink back on cooling and will crack on top. Nigella makes a sticky almond topping to fill the centre of the cake once it has cooled.
If the cake broke on side then it could be that the cake was not quite baked enough. With this type of cake it can be tricky to judge but the top should be firm, without any "wobble" in the centre if you jiggle the pan slightly. And a cake tester inserted into the centre should have damp crumbs attached but not unbaked batter. It is also possible that the cake pan was not quite greased properly. If the cake sticks to the side of the pan then the edges could break or crack as the cake sinks and pulls inwards. You could carefully run a thin-bladed knife around the inside edge of the pan when it comes out of the oven, to make sure that the edges of the cake are not sticking.