Prepare the Cookie Dough

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper for easy cleanup.
2. Mix the Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, cinnamon, nutmeg, and salt. Set aside.
3. Cream the Butter and Sugar: In a separate large bowl, cream together the softened butter and sugar until light and fluffy, about 2-3 minutes.
4. Add the Wet Ingredients: Beat in the egg, vanilla extract, and eggnog until well combined.
5. Combine with Dry Ingredients: Gradually mix in the dry ingredients until just incorporated. The dough will be soft and slightly sticky.

Shape and Bake the Cookies

1. Scoop the Dough: Use a cookie scoop or tablespoon to drop rounded portions of dough onto the prepared baking sheet, spacing them about 2 inches apart.
2. Bake the Cookies: Bake for 10-12 minutes or until the edges are golden and the cookies are set. Remove from the oven and allow them to cool on the sheet for a few minutes before transferring to a wire rack to cool completely.

Make the Milk Soak

1. Combine the Milk Ingredients: While the cookies are baking, whisk together the whole milk, heavy cream, eggnog, and sweetened condensed milk in a small bowl until well blended.

Soak the Cookies

1. Soak the Cookies: Once the cookies have cooled slightly but are still warm, carefully dip each cookie into the milk soak for 1-2 seconds, allowing the cookies to absorb the liquid. Be careful not to let them soak too long, or they may become too soggy.
2. Rest the Cookies: Place the soaked cookies back on the wire rack and let them rest for a few minutes to allow the milk soak to fully absorb.

Garnish and Serve

1. Add Whipped Cream: For extra indulgence, top each cookie with a dollop of whipped cream and a sprinkle of ground cinnamon.
2. Serve: Enjoy your Eggnog Tres Leches Cookies with a warm drink or as a festive treat for the holidays!