Measure the pumpkin puree, then spread it onto several layers of paper towels. Gently blot with additional paper towels to remove excess moisture. Set aside.

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

In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar on medium speed for 2 to 3 minutes, until light and creamy.

Add the egg yolk, blotted pumpkin puree, and vanilla extract. Beat until smooth and fully combined.

Add the dry ingredients in two additions, mixing on low speed only until no visible flour remains. Do not overmix.

Cover the bowl and refrigerate the dough for at least 1 hour.

Preheat the oven to 350°F / 175°C. Line two baking sheets with parchment paper.

In a small bowl, combine the granulated sugar, cinnamon, and optional pumpkin pie spice for the coating.

Scoop about 1 1/2 tablespoons of chilled dough and roll it into a ball. Roll the dough ball in the cinnamon-sugar mixture until completely coated.

Place the dough balls 2 inches apart on the prepared baking sheets.

Bake for 10 to 12 minutes, until the edges are set and the centers still look slightly soft.

Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.