In a large mixing bowl with an electric mixer, cream the butter and sugar together on medium-high speed until very pale and fluffy (about 3 minutes).
Scrape the bottom and sides of the bowl with a silicone spatula and add the egg, mixing until smooth.
Stir in the egg yolk and vanilla until smooth.
Add the flour, cornstarch, cream of tartar, baking soda, salt, and cinnamon, and mix on medium-low speed until a dough forms (it might look dry and sandy at first but if you keep mixing it will come together).
Place cinnamon sugar coated dough balls on the prepared baking sheets and bake for 6 to 10 minutes, or until set around the edges but still soft towards the middle.
Video
Notes
*If you don't have or don't want to use cream of tartar, it can be omitted. Just replace the baking soda with 3/4 teaspoon of baking powder.A few more of my favorite soft cookie recipes: