In a mixing bowl, sift the dry ingredients (flour, baking soda, cream of tartar, pumpkin pie spice, corn starch, and salt) together. Set aside.
In a stand mixer or a large mixing bowl with a hand mixer, beat the butter, sugar, and brown sugar until fluffy and well mixed. This should take 2-3 minutes. Remember to pause and scrape down the sides of the bowl.
Add the vanilla extract and pumpkin puree to the butter mixter. Beat for another minute until well mixed.
Slowly add the dry ingredients to the wet ingredients, mixing on low speed until combined into cookie dough. Cover the bowl and chill in your fridge for at least 1 hour, or up to overnight.
When the dough is chilled, preheat the oven to 350°F/180°C, and line 2 cookie sheets with parchment paper or silicone baking mats.
Mix together the sugar and cinnamon for the topping.
Then, with a cookie scoop, portion the chilled cookie dough into 2-tablespoon sized balls. Roll them with your hands to smooth, and then roll each ball through the cinnamon sugar mixture before placing them on the cookie sheets, at least 2 inches apart.
Bake in the preheated oven for 10-12 minutes, or until the edges are just turning a light golden color. Let the cookies cool on the trays for 5 minutes, then transfer to a wire rack to cool completely.
Notes
Substitutions: You can substitute vegan butter for the butter in this recipe. We haven't tested this recipe with gluten-free flour, but I expect that it will work well with your favorite all-purpose gluten-free baking flour blend.
For the Cinnamon Sugar Coating: feel free to add a pinch of pumpkin pie spice to the mixture for an extra warm, spicy flavor.
Chilling is Important: Snickerdoodles will spread too much if the dough isn't cold when you put it in the oven.
For even chewier pumpkin snickerdoodles, consider removing some of the water from your pumpkin puree. Just blot it between paper towels before adding it in. If you're using homemade pumpkin puree, you should definitely do this, since it normally contains more moisture than the canned version does.
Don’t overbake: Overbaked pumpkin snickerdoodles will be dry. Pull your cookies out of the oven just as they start to get brown. They will continue to cook a bit as they cool.
To Store: Keep in a sealed container at room temperature for up to 7 days, or freeze for up to 3 months.