Whisk dry ingredients. In a medium bowl, whisk the flour, cream of tartar, baking soda, cornstarch, and salt. Set aside.
Cream the butter and sugar. In a large bowl, beat the butter and sugar on medium speed until light and fluffy, about 2–3 minutes.
This traps air for a soft, puffy cookie.
Add eggs and vanilla. Beat in the egg, egg yolk, and vanilla until smooth and creamy.
Combine. Add the dry mixture in two additions, mixing on low just until the flour disappears. Don’t overmix.
Chill. Cover the dough and chill for 30–60 minutes. Chilled dough bakes thicker and chewier.
Preheat and prep. Heat the oven to 350°F (175°C).
Line baking sheets with parchment paper. Mix the rolling sugar and cinnamon in a small bowl.
Scoop and roll. Scoop 1.5-tablespoon portions (about a medium cookie scoop). Roll into balls, then coat generously in the cinnamon sugar.
Bake. Arrange 2 inches apart and bake 9–11 minutes, until the edges are set and the centers look slightly underbaked.
They’ll finish setting on the sheet.
Cool. Let cookies rest on the sheet for 5 minutes, then transfer to a rack. Enjoy warm or at room temperature.