1 Mix together the butter, sugar, and spices: In the bowl of a stand mixer fitted with a paddle attachment, place the butter, white sugar, brown sugar, baking powder, nutmeg, cinnamon and salt.
Mix on slow speed until most of the ingredients are blended, then increase to medium speed and mix until the butter is creamy and clings to the side of the bowl, about 2 minutes.
Scrape down the sides of the bowl halfway through mixing and then afterwards.
2 Whisk liquid ingredients: In a small bowl, add the eggnog, egg yolks, vanilla, and rum extract (if using).
Beat together with a fork then drizzle the mixture into the mixing bowl as it is turned on to low speed. Continue to mix on low speed until all the liquid is incorporated. Scrape down the sides of the bowl.
3 Beat in the flour: Add the flour and turn the mixer to low. Once most of the flour has been absorbed, increase the speed to medium low until no dry flour remains.
Scrape down the sides of the bowl once or twice to make sure the dough is completely homogenized.
4 Prepare the dough to chill: Scrape out the dough onto some plastic wrap and wrap tightly. Pat the dough down into a 1-inch thick layer. The dough will be sticky so, use the plastic wrap to help pat it down.
5 Chill the dough: Refrigerate the dough for at least 2 hours or overnight.
6 Preheat the oven and prepare the baking sheets: Once the dough has chilled, preheat the oven to 325°F.
Line two baking sheets with parchment paper or silicon baking mats.
7 Shape the cookies: Break off small chunks of the dough and roll them into 1-inch balls.
Place them on the baking sheet, about 2 inches apart.
8 Bake the cookies: Bake in the oven for 11 to 13 minutes, or until the bottoms are starting to turn golden brown.
9 Cool: Let the cookies rest on the warm baking sheet for 5 minutes before moving them to a wire cooling rack to cool completely.
Repeat with baking the remaining cookies.
10 Make the glaze: Once the cookies have cooled completely make the glaze by placing the sifted powdered sugar in a medium-sized bowl. Add the eggnog, vanilla extract, and rum extract (if using) and mix together.
11 Glaze the cookies: Drizzle the glaze over the cookies. Then garnish each cookie with a sprinkling of fresh grated nutmeg if you wish.
12 Serve with a big glass of real milk, and enjoy!