In a medium bowl, whisk 6 egg yolks and 2/3 cup sugar together until light and creamy.
In a saucepan over medium-high heat, bring 1 cup heavy cream, 2 cups whole milk, 1 teaspoon nutmeg, pinch of cinnamon, and a pinch of salt to a bare simmer.
Temper eggs. Add a spoon of hot milk into the egg mixture, whisking vigorously. Repeat, adding a spoonful at a time, to temper the eggs. When most of the hot milk has been added to the eggs, pour the mixture back into the saucepan on the stove.
Whisk constantly a few minutes until the mixture is just slightly thickened, or until it reaches 160℉ on a digital thermometer. Remove from heat and stir in the vanilla.
Pour through a fine mesh strainer into a bowl and cover with plastic wrap. Refrigerate until chilled. It will thicken as it cools. If you want a thinner, completely smooth consistency, blend with 1 or 2 tablespoons milk until smooth.
Top with whipped cream and a sprinkle of cinnamon or nutmeg, if desired.
Notes
Storage: Store homemade eggnog in the fridge up to 1 week.