I made Nigella's Old Fashioned Chocolate Cake from Feast (p269). When I checked the cake after 35 minutes of baking, there was a crack in the top and when I put the skewer into the centre it and it came out sticky, but in other parts of the cake the skewer came out perfectly clean. It seemed well baked on the whole. I let it cool under a cold fan. But the cake was very delicate, it was beaking up when I was shifting it from wire rack to plate. And after icing, when I cut it with a knife, again it was breaking into pieces. Taste wise it was very nice, soft, melting and moist. Everyone liked it. I didn't bake the cake in two separate tins, just in one tin and then cut it into halves using a thread. I baked it in a microwave convection oven. And I didn't have sour cream so I put the same quantity of single cream (thick straight from the fridge) and added some lemon juice to it. Why was the cake so fragile? Also what is the best substitute for sour cream here in Pakistan? Nazish.
We suspect that the cake was slightly under baked in the centre and that is probably the reason it was breaking up slightly. The recipe gives a baking time of 35 minutes for two sandwich-type cake tins. If you cooked all of the batter in one large tin then it will take longer to cook and you need to cook it until a skewer inserted into the centre comes out clean. In addition, a convection oven may need some temprature adjustment (Nigella's recipes have temperatures for conventional ovens) which your handbook should give guidance on. As convection ovens tend to cook more quickly without adjustments the edges of the cake could be done before the centre is properly cooked.
We would also suggest that you let the cake sit for about 10 minutes before using a fan to cool it as the cake will continue to cook and set when it first comes out of the oven.
For this cake recipe if you don't have sour cream then we would suggest using plain or natural yogurt instead. Use a full fat or whole milk yogurt and if possible use a strained yogurt, such as Greek yogurt. As yogurt has a lower fat content than sour cream we would also suggest you reduce the yogurt to 130g (1/2 cup) and increase the butter in the recipe by 25g (from 175g to 200g) to compensate.