Step 1: In a large bowl, using an electric mixer, cream together the softened unsalted butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.
Step 2: Beat in the room temperature egg yolk and vanilla extract until well combined. Scrape down the sides of the bowl as needed.
Step 3: Gradually add the eggnog to the butter mixture, mixing until just incorporated. The mixture might look slightly curdled, which is normal.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, ground nutmeg, ground cinnamon, baking soda, baking powder, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix the dough.
Step 6: Cover the bowl with plastic wrap and chill the dough in the refrigerator for at least 30 minutes, or up to 2 hours. This helps prevent spreading.
Step 7: Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat.
Step 8: Scoop rounded tablespoons of dough and roll them into balls. Place them about 2 inches apart on the prepared baking sheet.
Step 9: Bake for 10-12 minutes, or until the edges are lightly golden and the centers are just set (they may look slightly soft). Do not overbake.
Step 10: Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. Enjoy!