Soak the cherries for 2 hours prior to starting the recipe to reduce their potassium content. Dry the fruit, remove the cherry pits. Preheat the oven to 180°C (Th 6), butter an oven dish. Spread the fruit to cover the bottom of the pie pan. Mix the flour, sugar, and add a whole egg. Add 2 egg yolks, melted butter, milk and rum, mixing until it becomes a uniform mixture. Beat the egg whites until stiff and gradually add the mixture, and then pour over the cherries. Bake 15 minutes, then turn down the heat to 150°C (Th 5) and bake for another 20 to 25 minutes depending on your oven. Sprinkle vanilla sugar, enjoy warm or cold.