In the bowl of a stand mixer beat the butter, sugars, and and vanilla until light and fluffy, about 2 minutes. With mixing speed on low, add the egg and mix until combined.
In a medium size mixing bowl whisk together the flour, cornstarch, baking soda, cream of tartar, and salt. With mixing speed on low, gradually add the dry ingredients, mixing just until combined.
Using a standard size cookie scoop, form balls, smooth, and then roll into the cinnamon sugar mixture. Place on a cookie sheet and bake immediately or chill dough for 1 hour before baking.
Bake cookies at 350 degrees for 9-11 minutes, they will look soft, but as long as they are set on the edges they will firm up as they cool. Remove from oven and allow to cool on the sheets for 5 minutes before transferring to a wire rack to cool completely.
Notes
-Cornstarch is optional, but I like how it makes these cookies extra soft and chewy in the centers and crisp on the outside! -Sometimes I like to add a ¼ to ½ tsp. of cinnamon to the batter itself. That way you've got lots of cinnamon flavor!