Break graham crackers into smaller pieces, and place in a food processor; pulse until fine crumbs. Add melted butter to crumbs, and process until well blended. Press the graham cracker mixture into the bottom and up the sides of a pie pan. Freeze crust while preparing filling.
Make the filling:
In a heavy saucepan, combine marshmallows and eggnog over low heat. Cook, stirring constantly until marshmallows are melted and mixture is smooth. Remove from heat.
Stir in vanilla, salt, and eggnog flavoring. Cool 30 minutes, stirring several times.
While you’re waiting for the marshmallow mixture to cool, whip the heavy cream using a hand mixer or a stand mixer, until medium peaks form.
Fold whipped cream into the marshmallow mixture. Spoon the filling into the chilled crust, smoothing the top so it’s pretty. Sprinkle the top of the pie with nutmeg.
Refrigerate the pie, covered, 8 hours or overnight. Serve topped with whipped cream if desired.