In a medium bowl, combine dry ingredients: flour, cream of tartar, baking soda, pumpkin pie spice, and kosher salt. Mix well.

In a larger mixing bowl, whisk together melted butter, pumpkin purée, light brown sugar, granulated sugar, egg yolk, and vanilla extract until smooth.

Gradually add the dry mixture to the wet mixture, carefully folding with a rubber spatula until there are no flour patches left. The dough will feel soft.

Chill the dough by covering the bowl and placing it in the fridge for a minimum of 30 minutes to develop flavors and enhance texture.

Preheat your oven to 350°F and set two baking sheets lined with parchment paper on the top and bottom racks.

Prepare the sugar coating by whisking together 1/4 cup of granulated sugar with 1 teaspoon of pumpkin pie spice in a shallow dish.

Shape the dough into balls using a 1 1/2 tablespoon cookie scoop or a heaping tablespoon, rolling them smooth before coating in the sugar-spice mix. Space them 3 inches apart on the baking sheets.

Bake in the preheated oven for 11-12 minutes, rotating the sheets halfway for even baking—look for puffy cookies with cracked surfaces.

Let the cookies cool on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely. Store in an airtight container to keep fresh for up to 5 days.