Heat milk just until small bubbles form without boiling. Beat egg yolks with sugar until light and fluffy. Add salt and hot milk and mix well. Place over low heat or hot water and cook, while stirring, until mixture thickens (excess heat and long cooking will cause it to curdle).
Beat egg whites until stiff, gradually adding the 1/4 cup sugar. Fold whites into hot milk mixture. Stir in bourbon or rum. Serve hot with a dash of nutmeg.
Go back to the future for an eggnog recipe that doesn't involve any risk of raw-egg salmonella. From The Nashville Cookbook (1976, 1977 Nashville Area Home Economics Association), this recipe was contributed by Annie L. Foster and involves cooking both egg yolks and whites into a sippable, creamy custard. The original recipe called for scalding the milk, no longer necessary with today's pasteurized products, so this version merely calls for heating the milk in the first step.
Courtesy Annie L. Foster, from The Nashville Cookbook