Adjust oven rack to middle position; heat oven to 350° F. Line a 13 by 9-inch baking pan with foil, leaving enough overhang to help you lift the blondies from the pan when they're done. Turn the pan upside down, and form the foil over the bottom to make the shape of the pan with the foil. Turn the pan back over and press the pre-shaped foil in the bottom. Spray with cooking spray and set aside.
Whisk flour, baking powder, and salt together in medium bowl; set aside. Melt the butter over medium heat and allow to cool slightly. Whisk melted butter and sugar together in medium bowl until combined. Add eggs and vanilla and mix well. Using rubber spatula, fold dry ingredients into egg mixture until just combined; do not overmix. Fold in the assortment of mix-ins and turn batter into prepared pan, smoothing top with rubber spatula.
Bake until top is shiny, cracked, and light golden brown, 22 to 25 minutes; do not overbake. Cool on wire rack to room temperature. Remove bars from pan by lifting foil overhang and transfer to cutting board. Cut into 12 large or 16 small(ish) bars and serve.