Separate the eggs. Separate 6 large eggs, placing the yolks in a medium bowl and the whites in a large bowl (I recommend the 3-Bowl Method for this step). Cover the whites and refrigerate until needed, or freeze in an airtight container if aging the eggnog for longer than a day.

Whisk the yolks with the sugar. Add 1 cup granulated sugar to the egg yolks and whisk by hand or with a mixer with the whisk attachment on medium speed until the mixture is smooth, creamy, and lightened to a lemon-yellow color.

Whisk in the milk, cream, and liquor (if using). Add 2 cups whole milk, 1 cup heavy cream, and ½ to 1 ½ cups liquor if desired. Whisk until combined.

Cover and refrigerate. Cover the bowl and refrigerate for at least 1 hour. The more liquor you add, the longer it will keep — non-alcoholic eggnog should be consumed within 1 day; eggnog with ½ to 1 cup of liquor will keep for several days; and eggnog with 1 ½ cups of liquor will keep for several weeks and continue aging and thickening quite nicely. (If aging for longer than a few days, transfer the eggnog to a sealed glass container or a mason jar.)

Beat the egg whites into stiff peaks. Just before serving, whisk the reserved egg whites in a stand mixer with the whisk attachment or with a hand mixer at high speed until stiff peaks form.

Fold the egg whites into the eggnog. Transfer the beaten egg whites to the bowl with the eggnog and gently fold or stir the whites into the base — this gives the eggnog a frothy, extra-creamy texture. Some of the egg whites will also float to the top, like cappuccino foam.

Serve the eggnog. Transfer the eggnog to a pitcher or punch bowl. Serve in individual glasses with a grating of nutmeg over top.