Step 1: Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper or silicone mats.
Step 2: In a large bowl or stand mixer, cream together the softened butter, 3/4 cup granulated sugar, and brown sugar until light and fluffy (about 3-4 minutes).
Step 3: Add the egg and vanilla extract, beating until well combined and smooth.
Step 4: In a separate bowl, whisk together the flour, cornstarch, cream of tartar, baking soda, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 6: Chill the dough in the refrigerator for 30 minutes.
Step 7: In a small bowl, mix the 1/4 cup granulated sugar and 1 tablespoon of cinnamon together.
Step 8: Scoop about 1.5 tablespoons of dough and roll into a ball. Roll the ball thoroughly in the cinnamon-sugar mixture.
Step 9: Place the balls on the prepared baking sheets, spaced 2 inches apart. Flatten them slightly with your palm.
Step 10: Bake for 9-11 minutes, or until the edges are set but the centers still look slightly soft. Allow to cool on the pan for 5 minutes before transferring to a wire rack.