Preheat the Oven: Set your oven to 350°F (175°C). Line a baking sheet with parchment paper.
Mix the Wet Ingredients: In a large bowl, whisk together melted butter, brown sugar, and granulated sugar until smooth. Add pumpkin puree, egg, and vanilla. Stir until fully combined.
Combine the Dry Ingredients: In a separate bowl, whisk flour, cornstarch, cream of tartar, baking soda, pumpkin pie spice, cinnamon, and salt.
Form the Dough: Gradually fold the dry ingredients into the wet mixture until a soft dough forms. Chill for 30 minutes if the dough is sticky.
Shape and Coat: Scoop dough into small balls (about 1 ½ tablespoons each). Roll each ball in the cinnamon-sugar coating.
Bake: Arrange on the prepared baking sheet, spacing them about 2 inches apart. Bake for 10–12 minutes, or until edges are set and centers are soft.
Cool: Let cool on the baking sheet for 5 minutes before transferring to a rack. Enjoy warm or at room temperature.