In a large bowl or stand mixer, cream together butter and sugars. Beat until light and fluffy.
Mix in egg, followed by vanilla. Scrape bowl as needed.
Mix in salt, spices, baking powder and cream of tartar.
Add one cup of flour at a time, increasing speed after each addition.
Slowly add in eggnog, mixing until just combined and no streaks remain.
Chill dough for 1-2 hours or until dough is firm but manageable.
Preheat oven to 350°F. Line a baking sheet with a silicone mat or parchment paper.
In a small bowl, combine sugar, nutmeg and cinnamon for topping.
Using a small cookie scoop, scoop slightly heaping balls of dough. Shape into smooth 1-inch balls. Roll each in sugar mixture and space 2 inches apart on baking sheet.
Bake for 12-14 minutes. Cool on baking sheet for 5 minutes, then transfer to wire rack to cool completely.
Notes
Store in airtight container. Freeze for longer storage and thaw at room temp.