Preheat the oven to 350°F and grease a 9x13-inch baking pan with butter, then dust lightly with flour.

Melt the butter in a small saucepan over medium heat until it turns golden and smells nutty; take off the heat and let cool slightly.

In a medium mixing bowl, combine the flour, pumpkin pie spice, baking soda, and salt; whisk to combine.

In a large mixing bowl, pour in the cooled butter, then add the brown sugar, egg, pumpkin purée, and vanilla extract; whisk until fully combined and smooth.

Fold the dry ingredients into the wet ingredients until just incorporated. Add the white chocolate chips and chopped pecans, stirring to distribute evenly.

Transfer the batter to the prepared baking pan and spread evenly.

Bake for 35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs attached.

Allow to cool completely in the pan. Slice into squares and serve.