01 - Preheat the oven to 375°F. Line two baking sheets with parchment paper and set aside.
02 - In a medium bowl, whisk together the all-purpose flour, cream of tartar, baking soda, and salt until evenly distributed. Set aside.
03 - In a large mixing bowl, beat the softened butter and granulated sugar together using an electric mixer on medium speed until pale, light, and fluffy, about 3 minutes.
04 - Add the eggs one at a time, beating well after each addition. Mix in the pure vanilla extract until fully incorporated.
05 - Gradually add the dry ingredient mixture to the wet ingredients on low speed, blending just until the flour is fully incorporated. Do not overmix.
06 - In a small shallow bowl, stir together the granulated sugar and ground cinnamon until well blended.
07 - Scoop the dough by rounded tablespoonfuls and roll into even balls. Roll each ball thoroughly in the cinnamon-sugar mixture until completely coated.
08 - Place the coated dough balls on the prepared baking sheets, spacing them about 2 inches apart to allow for spreading.
09 - Bake for 9 to 11 minutes, or until the edges are lightly set and the centers appear just slightly underbaked for a soft, chewy texture.
10 - Let the cookies rest on the baking sheets for 2 minutes, then transfer to a wire rack to cool completely.