I'm just about to make one and I know it will have collapsed by the time it has cooled - they always do! They taste nice though and look fine when heaped with cream and fruit, it's just the centre cracks and collapses in.
I use Nigella's recipe - 4 egg whites, sugar, cornflour, vanilla extract plus a little white vinegar. It cooks for around 1-1.5 hrs (can't remember) and then sits in oven overnight.
Any ideas - is it the recipe? Thanks!