Preheat oven to 350ºF (177ºC). Line an 8" square baking pan2 with foil, leaving an overhang over the sides. Spray with non-stick spray. Set aside.
In a small bowl, mix together the flour, baking powder, baking soda, and salt. Set aside.
In a medium bowl, whisk together the melted butter, brown sugar, vanilla, and egg until smooth.
Slowly add the flour mixture to the wet ingredients and stir together until just combined. Fold in chopped candy bars. The batter will be quite thick. Spread batter evenly into prepared baking pan with a spatula (it will look like a very thin layer, but it will puff up substantially during baking).
Bake blondies for 18-20 minutes or until lightly browned on top and a toothpick inserted into the center comes out mostly clean. Allow to cool completely before lifting foil out of pan and cutting into squares. Blondies will stay fresh covered at room temperature for up to 5 days. Blondies freeze well, up to 3 months. Thaw at room temperature.
Candy: feel free to use any combination of leftover assorted chocolates you like, or use chocolate chips and/or nuts instead.
Baking pan: you can also put these candy bar blondies into an 8" round baking pan or springform pan and slice it like a cookie cake.