In a small bowl, whisk together egg yolks and sugar. Set aside.
In a stand mixer fitted with a whisk attachment, start beating egg whites on moderate speed until foamy. Add cream of tartar and increase speed. Beat until soft peaks form.
Heat milk in a medium saucepan over medium-low heat until it simmers. Drop a scoop of meringue into the milk and poach for 1-2 minutes. Flip meringue and poach another 1-2 min. Remove with a slotted spoon, place on a towel to drain, and set aside.
Take the simmering milk and slowly drizzle a little into the egg yolk sugar mixture, whisking constantly. Add the egg yolks to the saucepan of milk, whisk constantly, and place over medium-low heat. Cook for 5-10 minutes until the mixture thickens.
Remove from heat and pour in cold cream. For a thicker, more decadent eggnog, only use the egg yolks. Whisk in the remaining egg whites if you want a lighter, airier, foamy eggnog.
Grate freshly grated nutmeg or cinnamon into the eggnog.
