Brown the butter: Melt unsalted butter in a medium light-colored saucepan over medium heat. Swirl occasionally as it foams and sizzles. After 4 to 5 minutes, golden-brown specks will appear and a nutty aroma will rise. Immediately pour into a mixing bowl to cool for 15 minutes.

Blot the pumpkin: Measure 1/3 cup pumpkin puree and spread it over 3 layers of paper towels. Press gently with more towels to squeeze out excess moisture (this guarantees a chewy, not cakey, cookie texture).

In the bowl with cooled brown butter, whisk in the dark brown sugar and granulated sugar for 1 minute until smooth. Whisk in the blotted pumpkin puree, egg yolk, and vanilla extract until glossy.

Fold in the flour, baking soda, cream of tartar, pumpkin pie spice, and fine sea salt using a rubber spatula just until no dry flour streaks remain. Dough will be soft and pliable.

Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper. In a small bowl, whisk together the sugar and ground cinnamon.

Use a 1.5-tablespoon cookie scoop to portion dough into round balls. Roll each ball generously in the cinnamon sugar mixture until thoroughly coated, then place on baking sheets 2 inches apart.

Bake for 9 to 11 minutes until the edges are lightly set and tops develop signature crinkles. Centers should look slightly soft and underbaked.

Immediately bang the baking sheet gently on the counter to settle crinkles. Sprinkle with flaky sea salt. Allow cookies to rest on the hot pan for 5 minutes, then transfer to a wire rack to cool.