Whisk together eggs, sugar, and salt in a large mixing bowl.
Add to a medium-large saucepan set over low heat, then slowly whisk in 2 cups of the milk.
Cook egg/milk mixture for 18-22 minutes while stirring constantly, until it reaches 160°F and leaves a thin film coating the back of your spoon.
Remove the saucepan from the heat as soon as the eggnog mixture reaches 160°F. Immediately pour it into a large heat-resistant (not metal) bowl.
Stir in the vanilla extract, nutmeg, and cinnamon.
Very slowly pour in the remaining 2 cups of milk, stirring constantly.
Chill eggnog in an ice bath and/or transfer to the refrigerator to chill thoroughly.
Once chilled, stir in bourbon/rum/whatever type of alcohol you're using.
Add heavy cream to the bowl of a stand mixer and beat until soft peaks form.
Stir into the chilled eggnog until combined. Keep chilled, then garnish with nutmeg/cinnamon for serving.
