Chill two serving glasses in the freezer while you prep.
Add ice cream, eggnog, milk, vanilla, cinnamon, and nutmeg to a blender.
Blend until smooth and thick, about 20–30 seconds. Adjust with a splash of milk if too thick.
For a spiked shake, pour in rum or bourbon and pulse a few times.
Pour into chilled glasses. Top with whipped cream and a light sprinkle of nutmeg. Serve right away.