Step 1: Preheat your oven to 350°F (175°C) and line an 8×8 inch square baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a large mixing bowl, combine the melted butter and dark brown sugar. Whisk vigorously until the mixture is smooth and well combined.
Step 3: Add the pumpkin puree, egg yolk, and vanilla extract to the butter mixture. Whisk until fully incorporated and the batter looks uniform.
Step 4: Using a spatula, gently fold in the all-purpose flour, pumpkin pie spice, and salt. Mix just until no dry flour streaks remain, being careful not to overwork the dough.
Step 5: Fold in most of the chocolate chips, then spread the thick batter evenly into the prepared baking pan. Sprinkle the remaining chocolate chips over the top.
Step 6: Bake for 25 to 30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with just a few moist crumbs. Allow to cool completely in the pan before slicing.