Mexican eggnog made with egg yolks, vanilla, and rum is a festive holiday beverage. This family celebration drink is a rich, creamy, spirited tradition of Mexican Christmas.
In a heavy-bottom saucepan, combine whole milk and sweetened condensed milk. Heat gently over medium-low heat, stirring constantly to avoid scalding, until just steaming (do not boil).
In a separate bowl, beat the egg yolks until smooth and pale.
Temper the yolks by slowly adding about 1 cup of the hot milk mixture to the yolks, whisking constantly to prevent curdling.
Return the tempered yolk mixture to the saucepan. Continue to cook on low heat, stirring continuously with a wooden spoon, until slightly thickened and the mixture coats the back of the spoon (about 8–10 minutes). Do not let it boil.
Remove from heat. Stir in vanilla extract, ground cinnamon, and nutmeg. Let cool for 15 minutes before stirring in the dark rum. Chill thoroughly before serving. Serve cold, garnished with extra nutmeg or cinnamon if desired.