Preheat the oven to 350°F and line a cookie sheet with parchment paper.

In a large bowl, whisk together the flour, baking soda, baking powder, nutmeg, cinnamon, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using an electric hand mixer, cream together the granulated sugar, light brown sugar, and softened butter until the mixture is light and fluffy.

Add the egg and vanilla extract to the bowl and mix until well combined.

Pour half of the flour mixture into the wet ingredients and mix just until incorporated. Add the eggnog, then mix until combined, and finally, mix in the remaining flour mixture. Continue mixing until a soft dough forms.

Using a medium cookie scoop (about 2 tablespoons), drop balls of dough onto the prepared cookie sheet, spacing them 2 inches apart.

Lightly spray the bottom of a glass with non-stick cooking spray and use it to gently flatten the dough balls to about half of their original height, making them about ½ inch thick.

Bake for 9-11 minutes, or until the tops are no longer wet and the edges begin to brown.

Remove from the oven and allow the cookies to cool on the sheet for 2-3 minutes before transferring them to a wire rack to cool completely.

Once the cookies are fully cooled, prepare the glaze by whisking together the powdered sugar and eggnog until smooth and free of lumps.

Drizzle the glaze over the cooled cookies and let them sit until the glaze has set. The glaze will begin to firm up in about 15 minutes, and the cookies can be stacked after about 1 hour.