1. Brown the butter in a light-colored saucepan over medium heat, stirring frequently, until golden with browned bits on the bottom and a nutty aroma. Immediately pour into a heatproof bowl and let cool to just warm, about 15 minutes.

2. Whisk the cooled brown butter with the pumpkin puree until smooth, then whisk in the granulated sugar and brown sugar until glossy and well combined.

3. Add the egg yolk and vanilla extract and whisk until the mixture is smooth and slightly thickened with no streaks of yolk remaining.

4. In a separate bowl, whisk together the flour, cream of tartar, baking soda, salt, cinnamon, pumpkin pie spice, and cornstarch until evenly combined.

5. Add the dry ingredients to the wet ingredients in two additions, folding gently with a spatula just until no dry streaks remain and a soft, slightly sticky dough forms.

6. Cover the bowl and refrigerate the dough for at least 45 minutes and up to 24 hours to firm up and allow the flavors to develop.

7. When ready to bake, preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper. In a small bowl, stir together the coating sugar with cinnamon and pumpkin pie spice.

8. Scoop the chilled dough into 1 1/2 tablespoon portions, rolling each portion into a smooth ball. Roll each ball in the cinnamon sugar mixture to coat on all sides.

9. Arrange the dough balls on the prepared baking sheets, spacing them a few inches apart. Bake for 9–11 minutes, or until the edges are set and lightly golden while the centers still look slightly soft and puffy.

10. Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. Repeat with remaining dough, re-rolling in sugar if needed before baking.