In a small bowl, whisk together the dry ingredients. Set aside.
In the bowl of a stand mixer, cream together the butter and brown sugar until pale in color and fluffy, about 3 minutes. Scrape down the sides of the bowl. Add the molasses and mix until combined. Add the egg and vanilla extract and mix until fully incorporated, scraping down the sides of the bowl as needed.
With your mixer on low, slowly add the dry ingredients. Mix until the dough just comes together (do not over mix). Transfer the dough to a bowl and cover with plastic wrap. Place the dough in the refrigerator for at least 4 hours or overnight.
Preheat the oven to 350 degrees. Line a baking sheet with parchment or a silicone mat. Shape the dough into 1-inch balls. Place the dough balls 3 inches apart on the baking sheet. If you’re using sugar instead of glaze, roll the balls into the granulated sugar before placing them on the sheet. Bake for 10-11 minutes (no longer).
Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Once the cookies are completely cool, drizzle with the eggnog glaze.
Store the cookies in an airtight container for up to 1 week.
Add the vanilla extract and eggnog; whisk until smooth.