Brown the butter by melting unsalted butter in a saucepan over medium heat, stirring until golden brown, about 5-7 minutes.

Mix the cooled brown butter with dark brown sugar and granulated sugar until it resembles wet sand. Add egg yolk, pumpkin puree, molasses, and vanilla, mixing until well combined.

Whisk together flour, baking soda, cream of tartar, cinnamon, nutmeg, and salt in a separate bowl. Gradually add to wet mixture, mixing until no dry patches remain. Chill dough for 30 minutes.

Prepare cinnamon sugar by mixing granulated sugar and cinnamon in a bowl.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

Shape the chilled dough into balls and roll in the cinnamon sugar mixture, spacing them on the prepared baking sheets.

Bake for 9-10 minutes until edges are set and tops crack slightly. They should remain soft in the center.

Cool on baking sheets for 5-10 minutes before transferring to a wire rack.