In a medium bowl, whisk together flour, baking soda, salt, cinnamon, and nutmeg. Set aside.
Beat butter and sugars on medium speed until pale and fluffy (approx. 2-3mins).
Reduce speed and add egg yolk, eggnog, and rum. Beat until well combined.
Add flour mixture and mix until just combined.
Refrigerate cookie dough for at least 3 hours up to overnight.**
Preheat oven to 350°. Line baking sheets with parchment paper or silicone mats.
Using a medium cookie scoop (1 1/2 Tbsp), portion cookies onto your baking sheets. I did 10 cookies per sheet.
Bake for about 9-11mins or until golden on the edges. Remove from oven.
Let cool on pans for 2 minutes, then transfer onto wire racks to cool completely. ***
Rum & Eggnog Glaze:
Stir all ingredients with a fork until well combined. Drizzle over cooled cookies and sprinkle with nutmeg if desired.
Notes
* If you'd like to double the recipe, you can use one whole egg rather than 2 yolks.** I scoop my cookies before chilling, place them on a baking sheet, and pop the sheet into the fridge for a couple hours. I find it's easier than trying to portion cold dough and you won't need to chill as long. I also roll them into more rounded balls once chilled.*** The cookies are cakey when still warm but will settle and deflate into chewy (slightly cakey) cookies.RECIPE REVISED DEC 16, 2020 - Some of you were experiencing super flat cookies with the original recipe so I've revised it and clarified some things.