Separate the egg yolks from the whites and place the yolks in a large mixing bowl.
Whisk the egg yolks until they become slightly lighter in color.
Gradually add the sugar to the egg yolks, whisking continuously until the mixture is well combined and slightly thickened.
In a separate saucepan, combine the whole milk and heavy cream. Heat the mixture over medium heat until it’s just about to simmer. Do not let it boil.
Slowly pour about 1 cup of the hot milk mixture into the egg yolk mixture, whisking constantly. This tempers the eggs, preventing them from scrambling.
Gradually add the tempered egg mixture back into the saucepan with the rest of the hot milk mixture, whisking continuously.
Cook the mixture over medium heat, stirring constantly with a wooden spoon or heat-resistant spatula. Continue cooking until the mixture thickens slightly and coats the back of the spoon. Do not let it boil; you’re aiming for a custard-like consistency.
Remove the saucepan from the heat. Stir in the vanilla extract and ground nutmeg. Adjust the amount of nutmeg to your taste.
Allow the eggnog mixture to cool to room temperature. Then, cover the bowl or transfer the mixture to a container and refrigerate for at least 4 hours or overnight.
If you’re adding bourbon or rum, stir it into the chilled eggnog base.
Pour the eggnog into glasses. Optionally, top each serving with a dollop of whipped cream and a sprinkle of nutmeg.
Serve and enjoy your homemade eggnog!