In a saucepan over medium heat, combine the whole milk, heavy cream, and sugar. Stir frequently until the sugar dissolves and the mixture is heated through, about 5 minutes. Do not let it boil.
In a large mixing bowl, whisk together the egg yolks until they are thick and pale in color.
Gradually pour the warm milk mixture into the egg yolks while continuously whisking. This will temper the eggs and prevent them from scrambling.
Once combined, pour the mixture back into the saucepan and cook over low heat, stirring continuously until the mixture thickens slightly and coats the back of a spoon, about 5-7 minutes.
Remove from heat and stir in the vanilla extract, ground nutmeg, dark rum, bourbon, brandy, and salt. Mix well to incorporate all the flavors.
Let the eggnog cool to room temperature before refrigerating it for at least 2 hours or until cold.
Serve in glasses, garnished with a sprinkle of nutmeg.