Alcohol: If you want to add alcohol to your eggnog, start with ¼ cup brandy, bourbon, rum or whisky added at the same time as the vanilla, or after cooling the eggnog. Add more to taste, if desired.
Whisk the egg yolks and sugar together in a medium bowl 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 degrees F on a thermometer). It will thicken more as it cools.
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 and 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.