In a medium bowl, whisk egg yolks and sugar.
In a medium saucepan, mix heavy cream, milk, vanilla, cinnamon, nutmeg and salt. Stirring often, bring to a very slight simmer over medium heat.
Temper egg yolks by drizzling the hot milk mixture into the bowl of yolks and sugar while whisking constantly. Use a ladle to make drizzling easier. After adding about ½ of the hot milk mixture, pour the egg yolk mixture into the saucepan while whisking.
Whisking constantly, cook over medium heat until slightly thickened and able to coat the back of a spoon (160°F on your candy thermometer), 2-3 minutes. Do not bring to a boil.
Strain eggnog through a mesh strainer to remove any bits of egg. Cover and refrigerate.
If desired, blend until smooth using an immersion blender.
Stir before serving to mix up any spices that have settled to the bottom. Serve chilled, topped with whipped cream and a dusting of cinnamon, if desired.
For an adult beverage, add alcohol to each cup to taste.