Line two baking sheets with silicone baking mats or parchment paper. Set aside.

Place the butter, granulated sugar, and brown sugar in a medium mixing bowl. Use a hand mixer to beat on medium speed until well combined, about 1 minute.

Add the egg, egg yolk, and vanilla extract. Continue to beat until smooth, about 30 seconds.

Add the flour, cornstarch, baking soda, cream of tartar, and salt to the bowl. Beat on low speed, scraping the sides of bowl with a rubber spatula as needed, until a smooth dough forms.

Divide the cookie dough into 30 balls, using roughly 1 1/2 tablespoons of dough for each, and arrange them 1 1/2 inches apart on the baking sheets. Transfer sheets to freezer for 10-15 minutes. Alternately, place the dough balls on a small baking sheet or large plate to freeze, and transfer to the prepared baking sheets before baking.

Preheat the oven to 350°F.

Place the sugar, cinnamon, and salt in a small mixing bowl. Whisk to combine.

Remove the baking sheets from the freezer. Roll each cookie in the cinnamon sugar, creating a thick coating, and return to the baking sheets.

Bake the cookies on middle rack of oven until puffy and golden brown at edges, 15-17 minutes.

Check to see that cookies are done. Remove from oven or add time as needed.

Allow the cookies to rest on the sheets for 5-10 minutes, then transfer to a wire rack to cool completely before serving.