In a big casserole with thick bottom, bring to boil the maple syrup. Reduce heat and let simmer 5 minutes.
Meanwhile, in a bowl, dilute cornstarch in water. Add heavy cream and mix. Pour the cream mixture in one shot in the casserole and let simmer, stirring, until mixture has thickened. Remove the casserole from the heat. Add rum and stir. Serve with fresh fruits.