In a heavy saucepan, whisk eggs, sugar and salt. Gradually add 4 cups of the milk. Cook and stir fairly frequently over low heat until temperature reaches 165°. This can take 30-35 minutes. Do not allow to boil. Immediately transfer to a large bowl.
Stir in vanilla, nutmeg and remaining milk.
Place bowl in an ice-water bath, stirring until milk mixture is cool. If mixture separates, you can process it in a blender or food processor until smooth.
Refrigerate, covered, until cold, about 3-4 hours.
When ready to serve, beat the heavy cream until soft peaks form. Whisk the whipped cream gently into the cooled milk mixture. If desired, sprinkle with additional nutmeg before serving.
Be sure to use a candy thermometer or other kitchen thermometer to ensure you reach the right temperature.
Keep an eye on the mixture, you don't want to scorch the milk.
Be sure to plan ahead. The eggnog needs to chill for at least three hours.