Combine the egg yolks and sugar in a large mixing bowl and whisk until well combined.
Heat the milk, cream, and nutmeg over medium heat until they begin to steam, but do not boil.
Whisk constantly until everything is combined before adding the hot milk mixture to the egg yolk mixture.
The mixture should thicken and coat the back of a spoon after heating over low heat, stirring constantly. There is a risk of curdling the eggs if it is allowed to boil.
Stir in the bourbon and vanilla extract after removing the mixture from the heat.
Separately, beat the egg whites until stiff peaks form.
The egg whites should be gently folded into the eggnog mixture until everything is combined.
Refrigerate the mixture for at least 2 hours, or even overnight.
Garnish with whipped cream and nutmeg, if desired.