In a large bowl, whisk together the flour, cream of tartar, baking soda, cinnamon, nutmeg and salt.
In another 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.
Heat the oven to 350 F.
Line a baking sheet with parchment paper.
Form the dough into approximately 1-inch dough balls and roll in cinnamon sugar mixture. Place on the prepared baking sheet, about 2 inches apart.
Bake for 8 to 9 minutes, until slightly golden on the edges but still soft in the middle. Don’t overbake! These might 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 to 3 months.