01 - Preheat the oven to 375°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together the flour, cream of tartar, baking soda, and salt. Set aside.
03 - In a large bowl, beat the softened butter and sugar together using an electric mixer until light and fluffy, about 2 to 3 minutes.
04 - Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until evenly incorporated.
05 - Gradually add the dry flour mixture to the wet ingredients, blending on low speed just until combined. Do not overmix.
06 - In a small shallow bowl, stir together the 1/4 cup sugar and ground cinnamon until evenly mixed.
07 - Scoop the dough by rounded tablespoonfuls and roll into 1-inch balls. Roll each ball thoroughly in the cinnamon-sugar mixture to coat completely.
08 - Place the coated dough balls about 2 inches apart on the prepared baking sheets to allow for spreading.
09 - Bake for 9 to 11 minutes, or until the edges are set and the centers appear just slightly underbaked for a soft texture.
10 - Let the cookies rest on the baking sheets for 2 minutes, then transfer to a wire rack to cool completely.