Step 1: Melt the butter in a medium saucepan over medium heat. Whisk constantly until the butter foams, then turns golden brown with a nutty aroma. Remove from heat and pour into a bowl to cool for 15-20 minutes.
Step 2: Place the pumpkin purée on several layers of paper towels and blot firmly to remove as much moisture as possible until you have about 1/4 cup of concentrated purée remaining.
Step 3: In a large mixing bowl, whisk together the cooled brown butter, brown sugar, and granulated sugar until well combined and no lumps remain.
Step 4: Add the blotted pumpkin, egg yolk, and vanilla extract to the butter mixture. Whisk vigorously for 1-2 minutes until the mixture looks smooth and shiny.
Step 5: In a separate bowl, sift together the flour, baking soda, salt, and all spices. Gradually add the dry ingredients to the wet ingredients, stirring with a spatula until just combined.
Step 6: Fold in the chocolate chips. Cover the dough and refrigerate for at least 30 minutes (or up to 24 hours) to allow the flavors to meld and the dough to firm up.
Step 7: Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper.
Step 8: Scoop 1.5-tablespoon sized balls of dough onto the prepared sheets, spaced 2 inches apart. Bake for 10-12 minutes or until the edges are set. Allow to cool on the pan for 10 minutes before transferring to a wire rack.