Preheat the oven to 350F and line two large baking sheets with parchment paper. 

Add the softened butter, light brown sugar, and granulated sugar to the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until light and fluffy– about 1 minute. 

Next, add the pumpkin puree, egg yolk, and vanilla extract. Mix to combine. Scrape down the sides of the bowl as needed. 

In a small bowl, whisk together the flour, cream of tartar, pumpkin pie spice, and baking soda. 

Add the dry ingredients to the wet and mix just until no large lumps remain– you don’t want to overmix your cookie dough! 

To roll the cookies, add the granulated sugar and pumpkin pie spice to a small bowl. Scoop out 1.5 tablespoon sized portions and roll them into smooth balls. Roll the cookie dough in the sugar and then place the ball on the baking sheet. Make sure to place the cookies at least 2” apart as they will spread during baking. 

Press down on the cookies very gently to form a 1” thick disc. 

Bake for 10-12 minutes, or just until the cookies are set around the edges. 

Remove the cookies from the oven and allow them to cool completely on the pan.