Preheat the oven to 400 degrees F and line baking sheets with parchment paper.

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).

Combine the granulated sugar,

Scoop 1.5-tablespoon balls of snickerdoodle cookie dough and roll in the coating mixture.

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.