Blot the pumpkin: Place pumpkin purée between several layers of paper towels. Press gently to remove excess moisture until the purée reduces to about ¼ cup. Set aside.

Cream butter and sugars: In a large bowl, beat softened butter, granulated sugar, and brown sugar with a hand mixer for 1 to 2 minutes, until creamy and combined.

Add wet ingredients: Add the egg yolk, vanilla extract, and blotted pumpkin purée. Mix until smooth, scraping down the bowl as needed.

Mix dry ingredients: In a separate medium bowl, whisk together flour, cream of tartar, baking soda, salt, pumpkin pie spice, and cinnamon.

Make the dough: Add the dry ingredients to the wet ingredients. Mix on low speed just until combined. Do not overmix.

Chill: Cover the bowl and refrigerate the dough for 30 minutes.

Prepare for baking: Preheat the oven to 350°F. Line two baking sheets with parchment paper. In a small bowl, combine the coating sugar, cinnamon, and optional pumpkin pie spice.

Shape the cookies: Scoop the chilled dough into 1½-tablespoon portions. Roll each portion into a ball, coat thoroughly in cinnamon sugar, and place 2 inches apart on the prepared baking sheets.

Bake: Bake for 9 to 11 minutes, until the edges are set and the centers still look slightly soft.

Cool: Let the cookies cool on the baking sheet for 8 to 10 minutes. Transfer to a wire rack to cool completely.