I recently made the Coconut Cake from Nigella's Domestic Goddess book (p23). Both cakes were beautiful when they came out of the oven, but within 5 minutes they had collapsed down to virtually nothing! What did I do wrong?
The Coconut Cake is made with self-raising flour plus a little extra baking powder. Once of the potential problems could have been the extra baking powder. Nigella's primary method is to make the cake with a food processor and the extra baking powder is there to give the cake an extra lift as the food processor method doesn't incorporate as much air as a more traditional creaming method. If you made the cake with the creaming method and with the extra baking powder (the recipe does suggest to omit the extra baking powder in this case) then it could have risen a little too much - this stretches the glutens in the flour too far and then the cake collapses as it cools. Also if slightly too much extra baking powder is added then it would have a similar effect - you need a proper half teaspoon measure (2.5mls) and the baking powder should be a level half teaspoonful.
Alternatively it could be that the cake was not quite fully baked when it came out of the oven. It should be slightly shrinking away from the sides of the cake tins and firm to the touch on top. If the cake is removed too early then it will tend to collapse as it cools. Also as many ovens do not bake evenly it may be worth carefully switching the cake tins around after 20 minutes of baking to make sure the cake cooks evenly (though don't open the oven door too early as this will cause the cakes to collapse).