In a mixing bowl, sift the flour, baking soda, cream of tartar, pumpkin pie spice, cornstarch, and salt together. Set aside.
In a stand mixer fitted with a paddle attachment (or use a large mixing bowl and an electric hand mixer), beat the butter and both sugars on high speed until fluffy, about 2-3 minutes, stopping a couple of times to scrape down the sides of the bowl.
Add the vanilla extract and pumpkin puree to the bowl of butter and sugar. Beat on medium high for another 30-60 seconds to incorporate, scraping down the sides as needed.
With the mixer speed set to low, gradually add the dry ingredients to the wet ingredients, mixing until just combined. Cover the bowl of dough and chill in your fridge for at least 1 hour to prevent the cookies from spreading too much. If you're short on time, freeze it for 20 minutes.
When the dough is chilled, preheat your oven to 350°F. and line 2 baking sheets with parchment paper or silicone baking mats.
Mix the cinnamon, pumpkin pie spice, and sugar together in a shallow bowl.
Using a cookie scoop, measure out 2-tablespoon sized balls. Roll in the sugar mixture, then transfer to the prepared baking sheets 2-3 inches apart from each other.
Bake for 10-12 minutes, or until the edges are just turning a light golden color.
Remove from the oven and leave the cookies on the baking sheets for 5 minutes, then transfer to a wire rack to finish cooling. Enjoy!
Notes
Storage: Baked cookies will keep at room temperature for up to 7 days, or in a freezer for up to 4 months. The unbaked dough may be frozen for up to 3 months.
Substitutions: Swap all-purpose flour with gluten-free blend 1:1Substitute regular butter with dairy-free butter 1:1Unsalted butter: If you need to use salted butter, omit the salt called for in this recipe.