In a large bowl, whisk together the flour, cream of tartar, baking soda, cinnamon, nutmeg, and salt.
In a large bowl, beat together the butter, sugar, and brown sugar until light and fluffy, about 2 minutes.
Add the egg and beat until well combined.
Add the eggnog and vanilla, beating until combined.
Add the flour mixture, beating until combined, scraping down the sides of the bowl as needed.
Cover and refrigerate for at least 30 minutes, or up to overnight.
Preheat the oven to 350 degrees F.
Line a baking sheet with parchment paper. In a small bowl, mix togeher the sugar, cinnamon, and nutmeg.
Form the dough into approximately 1” dough balls and roll in cinnamon sugar mixture. Place on the prepared baking sheet, about 2” apart.
Bake in the preheated oven for 8-9 minutes, until slightly golden on the edges and still soft in the middle. Don’t overbake! These might still seem slightly underdone in the middle but they will set up.
Cool and enjoy!
Store in an airtight container for up to 1 week, or freeze for 2-3 months.