In a small bowl, whisk together the sugar and cinnamon. Set aside.
For the Cookie Dough:
Preheat the oven to 375°F. Line two baking sheets with parchment paper.
In a large bowl, whisk together flour, spices, baking soda, cream of tartar, and salt.
In the bowl of a stand mixer with the paddle attachment, beat butter on medium speed until smooth. Add the sugars and beat until light and fluffy, about 3 minutes.
Add the pumpkin purée, egg yolk and vanilla, and beat until combined, scraping down the sides as needed.
With the mixer on low, slowly add in the flour mixture. Mix just until combined. Cover and chill for at least 30 minutes or up to 24 hours. (Do not skip this step. The dough won’t roll in your hands if it isn’t chilled.)
Scoop 1½ tablespoons (32g) of chilled dough and drop it into the cinnamon sugar. Roll in the cinnamon sugar until well coated, then roll into a ball between your palms. Place balls 3 inches apart on the prepared baking sheets.
Bake for about 10 minutes or until puffed and golden on the edges. Let cool on the baking sheet for 5 minutes then transfer to a wire rack and cool completely.
Notes
Measure flour correctly: The easiest way to accurately measure flour is to weigh it using a kitchen scale. If you don’t have one, fluff the flour up in its container, spoon it into the measuring cup, and level off the top with a knife.
Use room temperature ingredients: Set the butter and egg out 30 minutes to an hour in advance so they soften and are easier to incorporate.
If the pumpkin puree appears watery, you can spread it on paper towels and let it sit to blot excess moisture. Excess moisture can lead to cakey cookies.
Chill the dough for at least 30 minutes. Do not skip this step. The dough won’t roll in your hands if it isn’t chilled. The cookies will also spread too much if warm.
For cookies that spread a bit more and bake up less puffy with crispier edges, bake them at 350°F for 14 minutes.
Let the cookies cool completely: This gives them time to set up perfectly so they develop that addictive chewy texture with crisp edges.