Whisk together the flour, cornstarch, baking powder, cinnamon, nutmeg, and salt in a medium bowl. In a separate bowl, whisk together the butter, eggnog, and vanilla. Stir in the granulated and brown sugars. Add in the flour mixture, stirring until barely incorporated. Chill the cookie dough for at least 30 minutes, or up to 3 hours. (If chilling longer than 3 hours, cover the cookie dough with plastic wrap to avoid letting it dry out.)

Preheat the oven to 350°F, and line a baking sheet with parchment paper or a silicone baking mat.

To prepare the coating, stir together the sugar, cinnamon, and nutmeg in a small bowl.

Using a spoon and spatula or a cookie scoop, drop one portion of cookie dough into rounded mounds in the bowl of spiced sugar. Rotate it until its fully coated; then roll it between your palms into a ball. Drop it back in the spiced sugar, coat again, and place on the prepared baking sheet. If the cookie dough was chilled longer than 1 hour, flatten the cookie dough balls slightly.

Bake the cookies at 350°F for 9-12 minutes. Cool on the baking sheet for 10 minutes before turning out onto a wire rack.