Pick over and wash prunes or dates and soak in cold water. Cook in same water until soft; then remove stones and rub through strainer. Add sugar and cook 5 minutes or until the consistency of marmalade. Beat whites until stiff and add when cold, fruit mixture gradually, and lemon juice. Heap lightly in buttered dish, and bake 20 minutes in a slow oven. Serve cold with thin custard or cream.