Preheat oven to 350 degrees Fahrenheit.
Whisk together flour, cream of tartar, baking soda, and salt. Set aside.
Using a mixer, cream butter, granulated sugar, and brown sugar until light and fluffy. Add egg and vanilla then beat well.
Gradually mix in the dry ingredients until evenly combined.
In a small bowl, combine cinnamon and sugar.
Roll dough into balls about one inch in diameter (tip: use a melon baller to scoop the dough into evenly sized balls). Roll the dough balls in the cinnamon sugar mixture until fully coated. Place on a baking sheet lined with parchment paper.
Bake for about 12 minutes, until set. Remove from the oven, let cool on the pan for about two minutes, then transfer to a wire rack to cool completely (or eat them right away!).
18 servings
1 cookie