Step 1: Preheat your oven to 350°F (175°C) and line an 8×8 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: Spread the 1/2 cup of pumpkin puree onto a plate and press firmly with several layers of paper towels to remove excess moisture until you have roughly 1/4 cup of concentrated puree left.
Step 3: In a large bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth and well combined.
Step 4: Add the blotted pumpkin puree, egg yolk, and vanilla extract to the butter mixture and whisk vigorously until the batter is glossy.
Step 5: Fold in the all-purpose flour, pumpkin pie spice, cinnamon, and salt using a spatula. Mix only until no streaks of flour remain.
Step 6: Gently fold in the white chocolate chips, then spread the batter into the prepared baking pan, smoothing the top.
Step 7: Bake for 28-32 minutes, or until the edges are golden and a toothpick comes out with a few moist crumbs. Let cool completely in the pan before slicing into 16 squares.