1. In a medium bowl, whisk together flour, baking soda, cream of tartar, pumpkin pie spice, cinnamon, and salt. 2. In a separate bowl, whisk melted butter, brown sugar, granulated sugar, pumpkin purée, egg yolk, and vanilla. 3. Gradually mix the dry ingredients into the wet mixture until a soft dough forms. 4. Cover and chill the dough for 30–60 minutes to firm up. 5. Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper. 6. Mix 1/4 cup sugar and 1 tsp cinnamon in a shallow bowl. 7. Scoop dough into 1.5-inch balls and roll in cinnamon sugar. 8. Place on baking sheet 2 inches apart and bake for 9–11 minutes. 9. Let cool on the sheet for 5 minutes, then transfer to a wire rack to cool completely.