Whenever I bake cakes or cookies the upper layer becomes very hard and sometimes burns. What should I do to prevent this?
There are a few potential reasons for cakes and cookies burning on top. The first is that if you are using a "multi function" electric oven that has heating elements on the top and bottom then you may need to check which function you are using on the oven. If the top element is running it may be causing the tops of the cakes or cookies to cook too quickly. It is probably best to refer to your oven manufacturer's handbook to check which function is best for baking. Also if you are using a fan oven then you need to adjust the oven temperature. Nigella's books, in common with most cookery books, gives conventional oven temperatures. Fan ovens need adjusting, this is usually done by reducing the oven temperature by 20c/50F from the conventional temperature, and again your handbook will give more detailed instructions.
Also check the positioning of the shelf in the oven. Cakes and cookies should be baked in the centre of the oven. If the shelf is too high then the tops may be cooking slightly too quickly. Finally, check that your oven is running to the correct temperature. An oven thermometer is inexpensive and useful for checking the temperature of the oven as sometimes oven thermostats can be slightly out.