Updated Nov 4, 2019Chatelaine
1. Position rack in centre of oven, then preheat to 350F. Line 2 large baking sheets with parchment.
2. Whisk flour with baking powder, cream of tartar, salt and nutmeg in a medium bowl. Beat butter with 1 cup sugar in a large bowl, using an electric mixer on medium, until fluffy. Beat in egg and eggnog until smooth. Reduce speed to low and gradually beat in our mixture until combined. Cover and refrigerate until chilled, about 1 hr.
3. Whisk 1/4 cup granulated sugar with cinnamon and 1/2 tsp nutmeg in a medium bowl. Scoop dough and roll into 1-in. balls. Place on a large plate and refrigerate at least 15 min. Roll balls in sugar mixture until coated. Arrange balls, 2 in. apart, on prepared sheets. (Cookies will spread as they bake.)
4. Bake until lightly golden and crisp around the edges, 11 to 12 min. Let stand on sheets for 1 min, then transfer to a rack to cool completely.
Cookies will keep well stored in an airtight container up to 3 days.