In a large bowl of a stand mixer, cream together the butter, granulated sugar and brown sugar together until creamy, about 20 seconds. Add in the egg yolk and beat well, followed by the pumpkin puree and vanilla extract.

In a large bowl, combine the flour, cornstarch, cream of tartar, baking soda, baking powder, salt, and spices and whisk well. Gradually add in the flour mixture to the stand mixer bowl with the wet ingredients, scraping down the sides of the bowl as needed, until a soft dough comes together. Place plastic wrap over the dough bowl and refrigerate for at least 1 hour or until easier to handle (dough is very sticky at first but will set more in the fridge during chilling time).

Preheat oven to 350 degrees F. Line 2 baking sheets with parchment paper or silicone liners. Meanwhile, in a small bowl, whisk together the topping: granulated sugar and cinnamon.

Scoop out about 1 Tablespoon of dough using a cookie dough scoop and roll it into a ball using your hands. Dredge the dough ball into the cinnamon sugar mixture. Place 2" apart on the baking sheets.

Bake for 12-14 minutes, rotating pans halfway through baking time to ensure even baking. Cookies may look slightly underdone; this is okay as they will continue to set up as they cool. Do not over-bake! Cool on the baking sheet for about 5-10 minutes before transferring to a wire rack to cool completely. If you have leftover cinnamon sugar after baking, I like to sprinkle some additional cinnamon sugar over the still-warm cookies for added flavor and texture.