Make the egg yolk mixture in advance and chill, so when you're ready for eggnog, it's ready for you. The recipe can be easily doubled for a big batch. Leftovers are fine for several days, kept refrigerated. Adapted from Chef Mary Sue Milliken in Saveur. The rum quantity can be adjusted down or up as you wish...
In a heatproof bowl set over a saucepan with just an inch or so of simmering water, stir the rum, sugar, and egg yolks. Cook, stirring constantly, until the sugar is dissolved and the mixture is very thick, 2–3 minutes. Chill for at least two hours, or set the bowl in an ice bath to cool it quickly.
Whip cream in a bowl until soft peaks form.
In a clean bowl, whip the egg whites until soft peaks form, adding a tablespoon of sugar while whipping.
Fold whipped cream and cold egg yolk mixture into the whipped whites; stir in the milk. Serve the eggnog from a punch bowl, the top of the eggnog dusted with nutment and/or cinnamon. Sprinkle more spice atop each individual glass of eggnog.