Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper leaving a 2-inch overhang along each side.
Combine white chocolate and butter in a heatproof bowl and place it over a saucepan with barely simmering water. Stir until completely melted and smooth. Or melt in the microwave at 50% power in short bursts with frequent stirring.
Combine brown sugar, egg yolks, pumpkin puree and vanilla in a large bowl and whisk until smooth. Mix in warm melted white chocolate mixture. Mix in pumpkin spice and salt. Add flour and fold it in gently. Just before all of the flour is incorporated, fold in the white chocolate chips.
Spread the rest evenly into prepared pan. Sprinkle extra white chocolate chips on top. Bake for 25-30 minutes until the top is golden, shiny and crackly, the edges are browned.