Preheat oven to 350F. Line an 8x8-inch metal pan with parchment, leaving a 2-inch overhang on two sides for easy lifting. Lightly butter any exposed pan surface.

Melt butter in a medium saucepan over medium heat, swirling occasionally. Continue cooking 3-4 minutes until the foam subsides and golden-brown bits form on the bottom. The butter will smell nutty and toasty. Immediately pour into a large heatproof bowl to stop cooking. Let cool 5 minutes.

In a separate bowl, whisk together flour, pumpkin pie spice, baking powder, and salt until evenly combined.

Whisk brown sugar into the cooled brown butter until thick and glossy, about 1 minute. Add pumpkin puree, egg, and vanilla; whisk until fully incorporated. The mixture will look slightly curdled at first but will smooth out.

Switch to a rubber spatula. Add dry ingredients and fold until just a few streaks of flour remain. Add white chocolate and butterscotch chips, folding until no dry spots remain. Do not overmix.

Scrape batter into prepared pan and spread to corners with an offset spatula. Bake 32-36 minutes until the top is set and a toothpick inserted in the center comes out with a few moist crumbs. The edges will be deeply golden and slightly pulled from the pan. Cool completely in pan on a wire rack, at least 2 hours.

Use the parchment overhang to lift blondies onto a cutting board. Slice into 16 squares with a sharp knife, wiping clean between cuts. Store airtight at room temperature up to 4 days.