Preheat oven to 350°F. Lightly grease a 9×13 inch pan with baking spray, and line it with parchment paper, leaving an overhang on the sides to lift the blondies out. Set aside.
In a large bowl, using a hand mixer, whisk the melted butter with white and brown sugar until combined.
Whisk in the peanut butter, the eggs, egg yolk, and vanilla extract. Whisk until fully combined.
Using a spatula fold in the flour, baking powder, and salt. The batter will be very thick. Fold in Reese’s Pieces and mini peanut butter cups.
Transfer the batter into the prepared pan and spread it evenly. If you have more Reese's pieces on hand sprinkle some on top.
Bake for 30 minutes, then test the blondies with a toothpick. Insert it into the center of the pan ad if it comes out with wet batter, bake longer, checking every 3 minutes. If there are only a few moist crumbs, the blondies are done. Do NOT overbake as they will taste a little dry if you do!
Remove from the oven and place on a wire rack to cool completely.
Once cooled, lift them out of the pan, cut into bars and serve.
Store leftovers in an airtight container at room temperature for up to 1 week.