Blot the pumpkin. Measure ⅓ cup pumpkin purée and gently press out some of its moisture with paper towels. Set it aside while you prepare the other ingredients.

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

Cream the butter and sugars. In a large bowl, beat the softened butter, granulated sugar, and brown sugar for about 2 minutes, until combined and a little lighter in color. Scrape down the sides of the bowl.

Add the pumpkin. Beat in the egg yolk, blotted pumpkin, and vanilla. The mixture may look slightly separated. That’s fine.

Finish the dough. Add the dry ingredients and mix on low speed just until no flour streaks remain. Don’t keep beating once the dough comes together.

Chill. Cover the bowl and refrigerate the dough for 45 to 60 minutes. It should feel firm enough to scoop and roll without sticking heavily to your hands.

Roll in cinnamon sugar. Heat the oven to 350°F (175°C). Line two baking sheets with parchment paper. Mix the coating sugar and cinnamon in a small bowl. Scoop the dough into balls of about 1 ½ tablespoons each, then roll each ball generously in cinnamon sugar. Place them about 2 inches apart.

Bake. Bake one sheet at a time for 10 to 12 minutes, until the edges look set and the tops no longer look wet. The centers should still feel soft. Let the cookies sit on the baking sheet for 5 minutes, then transfer them to a wire rack to cool.

You should get about 24 cookies, depending on the size of your scoop. Ovens and scoops vary, so use the look of the edges and centers as your final guide.