Preheat oven to 350F¹ (175C) and line cookie sheets with parchment paper
In stand mixer or with hand mixer cream butter and sugar together on medium-low speed until combined.
Add egg, egg yolk, and vanilla extract, stirring again until combined. Pause to scrape down the sides and bottom of the bowl as needed.
In separate bowl, whisk together flour, cream of tartar, cornstarch, baking soda, salt, and 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.
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.
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.