Melt Butter: Melt the butter and set aside to cool slightly, ensuring it remains liquid but not hot.
Combine Dry Ingredients: In a medium-sized mixing bowl, whisk together the flour, pumpkin pie spice, cornstarch, baking soda and salt. Set aside.
Combine Ingredients: In a large mixing bowl, pour the slightly warm butter, add both sugars, and whisk together until the mixture thickens slightly.
Add Egg: Whisk in the egg.
Mix in Pumpkin: Whisk in the pumpkin puree until smooth.
Incorporate Dry Ingredients: Add the flour mixture and stir with a silicone spatula or wooden spoon until you have a thick cookie dough.
Chill Dough: Refrigerate the dough for 45 minutes to firm up. If space is an issue, wrap the dough directly and chill without the bowl.
Preheat Oven & Prep Pan: While the dough chills, preheat the oven to 356ºF (180ºC) and line a baking sheet with parchment paper, set aside.
Prepare Sugar: Pour granulated sugar into a small bowl and set aside for coating the cookie dough.
Shape and Coat Cookies: Weigh out each cookie to about 35g (1.2 oz.) or use three level tablespoons per cookie. Roll each dough ball in the granulated sugar until well coated and place them on the prepared baking sheet, spaced well apart to allow for spreading. Do not flatten the balls.
Bake: Bake for 10-12 minutes until the edges are golden and the tops crack, while centers remain soft.
Cool: Tap the baking tray on a counter to deflate cookies slightly. Let them cool on the tray for 15 minutes before transferring to a wire rack to cool completely.