In separate bowl, whisk together flour, cream of tartar, cornstarch, baking soda, salt, and cinnamon.
3 cups all-purpose flour, 1 ½ teaspoon cream of tartar, 1 teaspoon cornstarch, 1 teaspoon baking soda, ½ teaspoon salt, ½ teaspoon ground cinnamon
With mixer on low speed, gradually stir flour mixture into your butter mixture until completely combined (the dough becomes fairly thick, and you will need to scrape the sides/bottom of the bowl several times to ensure it is well combined).
In a separate small bowl, stir together cinnamon and sugar for rolling.
¼ cup granulated sugar, 1 Tablespoon ground cinnamon
Roll dough into 1 ½ Tablespoon-sized balls and roll in your cinnamon/sugar mixture.
Place on prepared cookie sheets and bake on 350F (175C) for 11-12 minutes, the centers may appear slightly under-done still and that is OK! Allow cookies to cool completely on baking sheet where they will finish baking without drying out or becoming hard or crumbly.
Video
Notes
Never place cookie dough on hot cookie sheet, it will melt and you will have extremely thin cookies.
¹Recipe Update Note:
When I originally published this recipe I recommended baking the cookies at 375F (190C) for 10 minutes. That will still work (and work well!), but I've found that baking them at 350F (175C) for 11-12 minutes yields even softer, chewier cookies and now recommend that.