Prep oven and pan. 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. Spray with cooking spray and set aside. (you can also use parchment paper)
Combine dry ingredients. Whisk flour, baking powder, and salt together in medium bowl. Make a well in the dry ingredients.
Combine wet ingredients. In a second bowl, whisk together the melted butter, sugars, vanilla, egg, and peanut butter.
Combine wet and dry ingredients. Using rubber spatula, fold dry ingredients into wet mixture until just combined; do not overmix.
Add mix-ins. Fold in Reese's candies and chocolate chips until even mixed. Spread batter into prepared pan, smoothing top with rubber spatula. I like to sprinkle on a few extra chocolate chips and Reese's pieces for presentation.
Bake. Bake until top is shiny, cracked, and light golden brown, 22 to 25 minutes; do not overbake. Bars may look slightly underdone in the middle. Cool on wire rack to room temperature. Remove bars from pan by lifting foil overhang and transfer to cutting board. Cut into bars and serve.