Preheat the oven to 350°F (177°C). Adjust oven rack to the center rack position. Line the bottom and sides of a 9×13-inch metal or glass baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out of the pan. Set aside.

In a large bowl, whisk the melted butter and brown sugar together until combined. Whisk in the peanut butter, then add the eggs, egg yolk, and vanilla. Whisk it all until completely combined.

Add the flour, baking powder, and salt. Fold the batter together using a rubber spatula. Batter is very thick. Finally, fold in the Reese’s Pieces. Spread evenly into prepared pan and sprinkle with a couple extra Reese’s Pieces on top (for looks), if desired.

Bake for 30 minutes, then test the blondies with a toothpick. Insert it into the center of the pan. If it comes out with wet batter, the blondies are not done. If there are only a few moist crumbs, the blondies are done. Keep checking every 2 minutes until you have moist crumbs. They taste a little dry if you over-bake!

Allow the bars to cool completely in the pan set on a wire rack. Once cool, lift the bars out of the pan using the paper overhang on the sides and cut into squares.

Cover leftover bars and store at room temperature for up to 1 week.