In a small bowl, add the flour, baking soda, cream of tartar, kosher salt and spices. Mix with a fork to combine evenly and set aside.
Spread the pumpkin puree onto a paper towel, use a few layers here. Place another paper towel on top and pat out some of the moisture. Lift up the paper towel underneath with the puree, hold it over a small bowl and squeeze to get out the excess water. Place the strained puree into a small bowl. There will be about 1/2 cup puree left. Set aside.
In a large bowl, add the melted butter and both sugars. Mix with a nonstick spatula to combine and add in the strained pumpkin puree. Add in the dry ingredients and mix to form a thick dough. Cover the bowl with cling wrap and chill for at least 2 hours.
Remove chilled dough from the refrigerator and preheat oven to 350°F.
Prepare 2 baking sheets with parchment paper.
In a small bowl combine the granulated sugar and pumpkin pie spice for rolling. Scoop 1.5-ounce balls of dough and roll each in the spiced sugar mixture before placing on the prepared baking sheet.
Place on the baking sheet, leaving 2 inches in between each cookie. Bake for 12–14 minutes, until cookies are puffed with set edges but look slightly underdone.
Remove from oven and let cool.
Cookies will stay fresh at room temperature, in an airtight container, separated by parchment paper for up to 3 days. Cookie dough can be frozen in an airtight plastic bag for up to 3 months.
