In a 2-quart mixing bowl, beat the egg yolks and brown sugar with a hand mixer fitted with the whisk attachment until thick and lightened in color (about 3 minutes).
Whip in the heavy cream, milk, booze, and vanilla extract until slightly frothy, then set aside.
In another 2-quart mixing bowl, whip the egg whites with a hand mixer fitted with the whisk attachment until they've reached soft peaks stage.
Gently fold the whites into the yolk mixture, then stir in the cinnamon and nutmeg.
Cover and refrigerate for at least 2 hours, but no longer than overnight.
Stir before serving. Serve cold with additional nutmeg and/or cinnamon for garnish, if desired.