Heat oven to 350°F. Line an 8-inch square pan with parchment and lightly grease.
Stir melted butter, brown sugar, and granulated sugar in a large bowl until glossy and smooth.
Whisk in pumpkin puree and vanilla until fully combined.
In a separate bowl, whisk flour, pumpkin pie spice, cinnamon, baking powder, baking soda, and salt.
Fold dry ingredients into the wet mixture just until no dry streaks remain. Do not overmix.
Stir in white chocolate chips and pecans, if using.
Spread batter into the prepared pan and smooth the top.
Bake 26–30 minutes, until the center is set and a tester comes out with a few moist crumbs.
Cool completely in the pan, then lift out and slice into 16 squares.