Blot pumpkin puree between paper towels to remove excess moisture. Measure ⅓ cup after blotting.
Melt butter in a saucepan over medium heat. Optional: cook 3 to 4 minutes until golden and nutty for brown butter. Cool 10 minutes.
Whisk cooled butter with granulated sugar and brown sugar until sandy. Whisk in blotted pumpkin, egg yolk, and vanilla until smooth.
In a separate bowl, whisk flour, cream of tartar, baking soda, salt, pumpkin pie spice, and cinnamon.
Add dry ingredients to wet ingredients. Stir just until combined; do not overmix.
Cover dough and refrigerate at least 30 minutes, up to 24 hours.
Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
Mix sugar and cinnamon in a small bowl. Roll chilled dough into 1 ½ tablespoon balls and coat fully in cinnamon sugar.
Place dough balls 2 inches apart on baking sheets. Flatten slightly with your palm.
Bake 9 to 11 minutes, until edges are set and tops are lightly cracked.
Cool on baking sheet 5 minutes, then transfer to a wire rack to cool completely.
Notes
Do not skip chilling the dough. It keeps the butter firm so the cookies hold their shape instead of spreading thin in the oven.Nutrition (per cookie, approximate): Calories: 130 kcal | Carbohydrates: 18g | Protein: 1g | Fat: 6g | Fiber: 0.5g | Sugar: 11g | Sodium: 75mgNutrition information is automatically calculated and should be used as an approximation only.