Begin by preheating your oven to 350°F (175°C). Line two baking sheets with parchment paper to prevent sticking and ensure easy cleanup.
In a medium mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, ground nutmeg, and ground cinnamon until evenly combined. Set this mixture aside for later use.
In a large mixing bowl, use an electric mixer to cream the softened butter and granulated sugar together. Mix on medium speed until the mixture is light and fluffy, about 3-4 minutes.
Add the egg to the creamed butter mixture, then mix in the vanilla extract and eggnog. Beat until everything is well combined, ensuring no lumps remain in the batter.
Gradually add the reserved dry ingredients to the wet mixture. Carefully mix just until the dough comes together; be cautious not to overmix as this can make the cookies tough.
Using a cookie scoop or a tablespoon, scoop out portions of the dough and drop them onto the prepared baking sheets. Make sure to leave about 2 inches of space between each cookie to allow for spreading.
Place the baking sheets in the preheated oven and bake for 10-12 minutes. The cookies should be lightly golden around the edges but soft in the center. Once baked, let them cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
In a small bowl, mix the powdered sugar with a few tablespoons of eggnog. Stir until you achieve a smooth, pourable icing. If the icing is too thick, add a little more eggnog; if too thin, add more powdered sugar until the desired consistency is reached.
Once the cookies have cooled completely, drizzle the icing over each cookie using a fork or piping bag. To finish, sprinkle a pinch of ground nutmeg and cinnamon on each iced cookie for an extra festive touch and delightful flavor.
Notes
For a festive touch, garnish with extra ground nutmeg and cinnamon.