Adjust oven rack to the middle position and preheat oven to 350℉.
Lay 2 sheets of aluminum foil perpendicular to each other in a 9-inch by 13-inch baking pan, and push foil into the corners and sides of the pan, with extra foil hanging over the edges of the pan. Grease foil.
In a bowl, whisk together flour, baking powder, and salt.
In a second bowl, whisk sugar and melted butter together until combined. Add eggs and vanilla and whisk.
Fold in flour mixture until just combined.
Fold in pecans, semisweet chocolate chips, and white chocolate chips.
Pour batter into prepared pan and smooth top. Bake for 22 to 25 minutes, rotating pan halfway through baking, until the top is shiny and cracked and feels firm to the touch.
Let blondies cool completely. Using foil overhang, lift blondies from the pan. Cut into bars and serve.
Notes
When combining the wet and dry ingredients, mix until just combined to avoid overworking the batter, which can result in dense blondies.
Make sure the melted butter is cooled and the eggs are at room temperature to ensure a smooth batter and even baking.
Halfway through baking, rotate the pan to ensure the blondies bake evenly, preventing over-browning on one side.
Look for a shiny, cracked top and a firm touch to know when the blondies are done. A toothpick inserted should come out with a few moist crumbs, not wet batter.