In a medium bowl, whisk the egg yolks and sugar together until light and creamy.
In a saucepan, over medium-high heat, combine the cream, milk, nutmeg and salt; stir often until mixture reaches a bare simmer.
Add a big spoonful of the hot milk to the egg mixture; whisking vigorously.
Repeat, adding a big spoonful at a time, to temper the eggs.
Once most of the hot milk has been added to the eggs, pour the mixture back into the saucepan on the stove.
Whisk constantly for just a few minutes, until the mixture is just slightly thickened (or until it reaches about 160°F on a thermometer).
Remove from heat and stir in the vanilla, and alcohol, if using.
Pour the eggnog through a fine mesh strainer into a pitcher or other container: cover with plastic wrap.
Refrigerate until chilled; it will thicken as it cools.
If you want a thinner, completely smooth consistency, you can add the entire mixture to a blender with 1 or 2 tablespoons of milk and blend until smooth.
Serve with a sprinkle of cinnamon or nutmeg, and fresh whipped cream, if desired.
Can be safely stored in a refrigerator for up to one week.
Notes
If you want to add alcohol to your egg nog, start with ¼ cup brandy, bourbon, rum or whiskey; this can be added at the same time as the vanilla or after cooling the eggnog: add more to taste, if desired.