Eggnog is essentially a dessert sauce called creme anglaise. The addition of flour to this recipe is unorthodox but it enables you to boil the sauce without curdling it. HEAT MILK, CREAM AND SUGAR in a saucepan and bring just to a boil. (Do not stir--leaving the sugar on the bottom of the pot will prevent the milk from scorching.) Beat the egg yolks and flour together with a wire whisk. Add about a cup of the hot milk mixture to the yolks and whisk to blend. Then whisk the yolk mixture into the mixture in the saucepan. Bring to a boil, stirring constantly. Boil gently for 10 seconds, then remove from heat and strain into a bowl, discarding the flour lumps. Chill and serve with rum, if desired.