Add nut milk, heavy cream, cinnamon, salt, and nutmeg to a medium saucepan or dutch oven. Simmer for about 8-10 to infuse. Remove from heat and set aside while you prepare the egg yolks. Don't allow the mixture to boil, or the milks will curdle.
Add egg yolks and sweetener to a large bowl. Mix or beat them together with a whisk or an electric mixer. Mix until light, fluffy, and most of the sweetener has dissolved. This should take about 2 minutes.
Gradually temper the hot milk mixture into the egg mixture. Tempering means adding the hot milks little by little to your egg mixture while mixing constantly, so the eggs don't curdle. Return everything to the dutch oven, and cook over medium-low heat for roughly 8-10 minutes, until the mixture coats the back of a spoon.
Remove the eggnog from the heat, and strain the mixture in a large mixing bowl through a metal sieve. Stir in the vanilla extract, and optional liquor of choice, to taste if using. Cover and transfer to the refrigerator to chill if you prefer to enjoy eggnog cold, you can also enjoy it hot.
Pour chilled eggnog into glasses, top with whipped cream if desired, along with extra nutmeg, and a cinnamon stick.
Store leftovers in an airtight container in the fridge for 2-3 days. Do not freeze.