Place 4 ramekins or custard cups into a shallow baking dish, and fill the dish with water to half-way up the sides of the ramekins.
Pour the eggnog into a pan over medium heat. Cook and stir occasionally until the mixture simmers, about 10 minutes.
Meanwhile, place the egg yolks and sugar into a mixing bowl; beat until light colored and frothy. Stir in the mascarpone until well blended and smooth.
Whisk 1/4 cup of the heated eggnog mixture into the eggs. Gradually whisk the remaining eggnog into the eggs.
Pour the mixture through a fine sieve to remove any egg strands. If desired, stir in the nutmeg, cinnamon, and vanilla.
Pour into the prepared ramekins, dividing evenly.
Bake in preheated oven until custard has set, 30 to 45 minutes. Centers should wiggle slightly when shaken, but not be soupy.
Remove from oven and cool 30 minutes; refrigerate at least 3 hours before serving.
