Melt butter in a light-colored saucepan over medium heat, swirling occasionally for 5-6 minutes until foam subsides, nutty aroma forms, and golden-brown flecks appear at the bottom. Pour into a heatproof bowl and let cool 15 minutes.

Spread pumpkin puree across paper towels on a plate, pressing gently with additional towels to absorb excess moisture until thick and reduced. Measure 1/2 cup blotted puree.

In a large mixing bowl, whisk cooled brown butter, brown sugar, and granulated sugar for 1 minute until well blended. Whisk in the blotted pumpkin puree, egg yolk, and vanilla extract until smooth.

Fold in all-purpose flour, cream of tartar, baking soda, pumpkin pie spice, and sea salt with a wooden spoon or rubber spatula just until combined (do not overmix). Cover and chill dough in refrigerator for 45 minutes.

Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper. In a small shallow bowl, whisk together granulated sugar, ground cinnamon, and nutmeg.

Scoop 2-tablespoon portions of chilled dough, roll into smooth balls with hands, then roll generously in the cinnamon sugar coating until completely covered. Place 2 inches apart on baking sheets.

Bake for 10 to 12 minutes until edges are set and tops are beautifully crinkled. Immediately tap baking sheet firmly on the counter to settle the crinkles. Cool on pan for 5 minutes, then transfer to a wire rack.