Peel and boil the sweet potatoes. Then mash or purée them while still hot. Separate the egg whites from the yolks. Whisk the egg whites until very stiff. In a bowl, mix the butter, sugar, egg yolks, rum, vanilla, lemon and puréed sweet potatoes. Add the egg whites then spoon everything into a buttered cake tin. Bake in a warm oven (150°C/300°F) for about 45 minutes.