Preheat oven to 375°F and line baking sheets with parchment paper.

In a bowl, whisk together flour, cream of tartar, baking soda, and salt until evenly combined.

In a separate mixing bowl, beat softened butter and granulated sugar with a mixer until light and fluffy, about 2–3 minutes.

Add the egg, egg yolk, and vanilla extract. Mix until smooth and fully incorporated.

Gradually add the dry ingredient mixture to the butter mixture while mixing on low speed until a thick dough forms.

In a small bowl, combine ¼ cup sugar and 1 ½ teaspoons cinnamon to create the coating mixture.

Use a medium cookie scoop to portion the dough. Roll dough into balls and coat completely in the cinnamon sugar mixture.

Place dough balls onto prepared baking sheets about 2 inches apart.

Bake for 10–11 minutes until edges are set and centers appear soft and slightly underbaked.

Allow cookies to rest on the baking sheet for 5 minutes, then transfer to a cooling rack to cool completely.