Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving overhang on two sides for easy removal. Lightly coat with cooking spray.

In a medium bowl, whisk together flour, cocoa powder, baking powder, and salt until no cocoa lumps remain. Set aside.

In a large bowl, whisk pumpkin puree, granulated sugar, brown sugar, eggs, and vanilla until completely smooth and well combined, about 1 minute. The mixture should look glossy and uniform.

Pour dry ingredients into wet ingredients. Use a rubber spatula to fold until just combined - do not overmix. A few small streaks of flour are okay. Fold in half the chocolate chips if using.

Spread batter evenly into prepared pan; it will be thick. Sprinkle remaining chocolate chips on top. Bake 22-26 minutes until a toothpick inserted in the center comes out with moist crumbs but no wet batter. The top should look set and slightly matte.

Cool completely in pan on a wire rack, at least 30 minutes. Use parchment overhang to lift out, then cut into 16 squares. These set up and taste better once fully cool - resist cutting early.