1Separate six eggs, reserving both yolks and whites.
2Combine egg yolks with one cup of granulated sugar.
3Mix the egg yolk and sugar mixture with a whisk or electric mixer until pale, thick, and fluffy, showing visible trails when the beaters move through.
4In a medium thick-walled pot, combine one cup of heavy cream and up to two cups of milk, along with a quarter teaspoon of freshly grated nutmeg.
5Heat the cream and milk mixture over medium heat until steamy but not boiling, whisking occasionally to prevent scalding.
6Temper the egg yolk mixture by whisking vigorously while slowly drizzling in half a cup of the hot milk mixture.
7Return the tempered egg yolk mixture to the pot with the remaining warm milk and cream, whisking constantly.
8Cook the mixture over medium-low heat, whisking constantly with a narrow whisk, until it thickens and reaches 160 degrees Fahrenheit.
9Strain the thickened eggnog through a sieve into a bowl to remove any curdled bits or large nutmeg pieces.
10Stir in at least one cup of bourbon, rum, or brandy, and one tablespoon of vanilla.
11Chill the eggnog in the refrigerator for at least three hours, or ideally a day or two, before serving.
12Just before serving, whip the six reserved egg whites on high speed until frothy, fluffy, and soft peaks form.
13Gently fold the whipped egg whites into the chilled eggnog mixture, preserving as many bubbles as possible.
14Ladle the eggnog into glasses and garnish with a sprinkle of cinnamon or nutmeg.