Brown the butter: In a light-colored pan, melt butter over medium heat. Stir continuously until golden brown specks form and it smells nutty. Remove from heat and cool for 10 minutes.

Mix wet ingredients: In a large bowl, whisk brown butter, brown sugar, and granulated sugar until combined. Stir in pumpkin, egg yolk, and vanilla.

Add dry ingredients: In a separate bowl, whisk together flour, baking soda, salt, cinnamon, pumpkin pie spice, and cream of tartar (if using). Slowly mix into the wet ingredients.

Chill the dough: Cover and refrigerate for 30–45 minutes until firm.

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

Make cinnamon sugar: Mix granulated sugar and cinnamon in a small bowl.

Form cookies: Scoop 2-tablespoon portions of dough, roll into balls, then coat each in cinnamon sugar. Place 2 inches apart on baking sheet.

Bake: 10–12 minutes, until edges are set and centers appear slightly underbaked.

Cool: Let rest on baking sheet 5 minutes before transferring to wire rack.

Serve & store: Serve warm or store in an airtight container at room temperature for up to 5 days.