Step 1: Prepare the Dough

1. In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar using an electric mixer on medium speed. Beat for about 2-3 minutes until light and fluffy.
2. Add the egg, eggnog, and vanilla extract to the butter mixture. Beat until well combined. The mixture may look slightly curdled—this is normal.
3. In a separate bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
4. Cover the dough with plastic wrap and refrigerate for at least 1 hour. Chilling the dough helps prevent the cookies from spreading too much during baking.

Step 2: Bake the Cookies

1. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper or a silicone baking mat.
2. Scoop tablespoon-sized portions of dough and roll them into balls. Place the dough balls about 2 inches apart on the prepared baking sheet.
3. Bake for 10-12 minutes, or until the edges are set and lightly golden. The centers may look slightly underbaked, but they will firm up as the cookies cool.
4. Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Step 3: Make the Eggnog Glaze (Optional)

1. In a small bowl, whisk together the powdered sugar, eggnog, and nutmeg until smooth. Add more powdered sugar if the glaze is too thin, or more eggnog if it’s too thick.
2. Drizzle the glaze over the cooled cookies using a spoon or a piping bag. Let the glaze set for 10-15 minutes before serving.