In a medium bowl, whisk together flour, pumpkin pie spice, cream of tartar, baking soda, and salt.
Combine melted and cooled butter, sugar, and brown sugar in a medium bowl. Whisk in egg, pumpkin puree, and vanilla. Continue mixing until combined. Add the dry ingredients to the butter mixture and fold together until combined and no flour streaks remain. Cover the bowl with plastic wrap and chill in the fridge for at least 30 minutes.
Preheat oven to 350°F and line 2 baking sheets with parchment paper.
Remove the cookie dough from the fridge and use a 1.5 tablespoon-size spring-loaded cookie scoop to scoop the dough balls. Roll each into a ball, coat in the cinnamon sugar mixture, and place on the prepared baking sheets, about 2 inches apart. Use your hand or the bottom of a glass to flatten the tops. Bake for 10-12 minutes or until the cookies brown around the edges and the center begins to set.
Let cool on the sheet for about 5 minutes before transferring to a wire rack to cool completely.
Notes
Storage: Once cooled, these pumpkin snickerdoodles will last for up to a week in an airtight container in the fridge or on the countertop! There's no need to reheat them, though!