Weigh out the ingredients
You'll need flour, butter, pumpkin purée, brown sugar, granulated sugar, eggs, pumpkin spice, vanilla, baking soda, salt, and cream of tartar.
Combine dry ingredients
In a medium or large mixing bowl, whisk together the flour, baking soda, cream of tartar, pumpkin spice, and salt. Set aside.
Melt the salted butter over medium heat, stirring it with a rubber spatula while melting. Once it turns a dark golden color, pour into a large bowl, scraping out the fat solids at the bottom, and weigh out to 170g (¾ cup).
Brown the butter
Fold up a few paper towels and place the pumpkin in the center. Take another set of paper towels and press the pumpkin flat to soak up the moisture. Repeat this several times until very little moisture comes away and the pumpkin weighs 113g (½ cup).
Dry the pumpkin purée
To the brown butter, whisk in the sugars until fully combined.
Whisk brown butter & sugars
Mix in the dried pumpkin purée, egg, egg yolk, and 2 teaspoons vanilla until smooth and no more lumps of pumpkin remain.
Mix in wet ingredients
Lastly, fold in the dry ingredients until fully combined. Cover the bowl and chill for one hour.
Add dry ingredients & chill
Use a 2 tablespoon cookie scoop to portion the cookie dough.
Portion the dough
Roll each cookie dough ball in spiced sugar and place on the baking sheet about 2 inches apart.
Roll in sugar
Bake the pumpkin snickerdoodles for 8-10 minutes or until just sightly underbaked. Allow to cool completely on the baking sheet.