Mix the coating sugar and cinnamon together in a small bowl. Preheat the oven to 400˚F and line two rimmed sheet pans with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar together for 3 minutes at medium speed, or until light and fluffy. Scrape down the sides of the bowl with a rubber spatula and add egg and vanilla. Mix at medium speed for about 1 minute.
In a medium bowl, whisk together flour, cream of tartar, baking soda, salt and nutmeg (if using). Add the dry ingredients to the butter mixture and mix at low speed until the flour has been incorporated.
Using a 1.5 tablespoon-size spring loaded cookie scoop, scoop 9 dough balls on each prepared baking sheet about 2 inches apart. Roll into even balls, then coat each in the cinnamon sugar mixture. Bake for 10-12 minutes, or until the cookies puff in the middle and set around the edges.
Let cool on the sheet for about 1 minute before transferring to a wire rack to cool completely.
Notes
Storage: Cool completely. Store in an airtight container at room temperature. They will keep well for up to a week. Tip: For fluffy, or thick Snickerdoodle cookies chill the dough in the refrigerator for an hour or so before baking.