At least 8 or up to 24 hours before serving time: In the bottom pan of a double boiler, heat an inch of water to a gentle simmer. In the top pan of the double boiler, whisk together egg yolks, sugar, and salt until well blended.
Gradually whisk in hot—120°F (49°C)—milk and nutmeg. Cook over simmering water, stirring constantly, until mixture begins to thicken and registers between 160-170ºF (71-77ºC); do not boil. It will not be as thick as a heavy custard!
Stir in cold milk. Strain custard into a 2-quart bowl.
Stir in the vanilla and the brandy or rum, if using. Cover and refrigerate until well chilled, at least 8 hours. (Or make a large ice bath and cool the 'nog in the bowl while stirring it.)
Whip cream until soft peaks form. With a wire whisk, gently fold/mix whipped cream into custard.
Pour eggnog into a chilled 2- or 2 1/2-quart punch bowl or pitcher; sprinkle with nutmeg. Serve!