In a stand mixer with the whisk attachment (or use a hand-held mixer), beat egg yolks on medium-high speed until they start to thicken and turn pale yellow, about 2 to 3 minutes. While beating, slowly stream in 1 cup sugar, until fully combined and the mixture forms a ribbon when beaters are raised. Beat in vanilla bean or extract until combined.

Turn mixer to low and add milk, a little at a time until blended. Mix in bourbon, rum and nutmeg. Remove to a large bowl.

Separately (you can use the same mixing bowl), beat heavy cream one high speed, with the remaining 1/4 cup sugar until medium-soft peaks appear. Fold the whipped cream into the egg mixture. Transfer to an air-tight storage container and let stand in refrigerator for at least several hours or up to a month (eggnog will mellow as it ages)

When ready to use, give it a stir, pour into serving glasses, sprinkle with nutmeg and enjoy.