In a small bowl, mix together flour, cornstarch, baking soda, cream of tartar, salt and cinnamon. Set aside.
In a large bowl mix together butter and sugars with a handheld mixer.
Add egg & vanilla and mix well
Gradually add in dry ingredients, mixing well. Continue until all the dry ingredients are well incorporated. Dough should feel soft but not sticky. If it's too wet to roll without sticking to your hands, add extra flour, 1 tbsp at a time.
Roll into balls, about 1 tbsp size. Roll into cinnamon sugar mixture. Bake for 8-9 minutes or until cookies are just slightly turning golden. They may still look slightly undercooked in the middle. This is fine, the cookies will continue to bake on the cookie sheet after removing from oven.
Let sit for 5 minutes, then remove to a wire rack to continue cooling.
Notes
Dough can be made ahead of time and chilled in the refrigerator. When you're ready to bake, allow dough to soften before baking (around 30 minutes sitting on the counter.)