Preheat your oven to 350°F (175°C) and line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal.
In a medium saucepan over medium heat, melt the unsalted butter. Continue to cook, stirring frequently, until the butter turns a golden brown color and smells nutty (about 5-7 minutes). Be careful not to burn it. Remove from heat and let it cool slightly.
In a large bowl, combine the brown sugar and browned butter. Stir until well mixed. Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
Fold in the toasted pecans and white chocolate chips (if using) until evenly distributed throughout the batter.
Pour the batter into the prepared baking pan and spread it evenly. Bake for 25-30 minutes, or until the blondies are set and a toothpick inserted into the center comes out with a few moist crumbs.
Allow the blondies to cool in the pan for at least 10 minutes before lifting them out. Let them cool completely on a wire rack before slicing into squares.
Notes
Serve warm or at room temperature, dusted with powdered sugar, and with vanilla ice cream.