Instructions:
- Combine dry ingredients: Whisk together the flour, baking powder, and salt in a medium bowl. Set aside.
- Cream the butter and sugar in a stand mixer on medium speed until light and fluffy (about 3 minutes). Scrape down the sides.
- Beat in the egg yolks, vanilla, nutmeg, cinnamon, and liquor (if using) until just combined.
- With the mixer on low speed, gradually add the flour mixture until just incorporated. Stop mixing the instant the flour streaks disappear.
- Drizzle in the cold eggnog and mix briefly until the dough just comes together. Do not overmix; the dough will look slightly crumbly.
- Divide the dough in half, form each into a flat disk, wrap tightly in plastic wrap, and chill for at least 90 minutes.
- Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper.
- Remove one dough disk, allow it to soften for 5–10 minutes, then roll the dough to a uniform thickness of about 1/4 inch (6mm) or scoop uniform balls.
- Place dough 2 inches apart on the prepared sheets. Bake for 10–12 minutes. Cookies should be set but still pale in the centre.
- Let cookies rest on the sheet for 5 minutes before transferring carefully to a wire rack to cool completely.
- To make the glaze, whisk together the sifted powdered sugar, nutmeg, and salt. Slowly add the milk/cream, starting with 2 tablespoons, until a smooth, pourable consistency is achieved.
- Once cookies are completely cool, drizzle or lightly spread the glaze over the top of each cookie. Allow the glaze to fully set (about 30 minutes) before storing.