In a heavy saucepan, heat milk until steaming (stirring frequently so it does not curdle). Whisk egg, egg whites and sugar together in a medium bowl. Gradually whisk the hot milk into the egg-sugar mix. Return the mixture to the saucepan and cook over medium heat. stirring constantly for 5-6 minutes, or until the mixture thickens enough to coat the spoon. Do not overheat or it will curdle. Remove from heat, pour through a strainer into a bowl. Stir in vanilla extract, cover and refrigerate until chilled or up to 3 days. To serve, add 1 T rum flavoring and 2 ice cubes to half of the mixture and blend in a blender until frothy. Pour into glasses and sprinkle with freshly grated nutmeg. Repeat with the second half of the mixture. Serves 6.
Note: Because the eggs have been cooked, this is safe to drink and to store, refrigerated for 2 or 3 days. Variations: For an alcohol-containing variation, add 1/4 c of dark rum, brandy or bourbon to the custard, instead of rum flavoring. For a higher calorie version, use whole milk or cream instead of skim milk (the cooking time will be a little longer).