Gather all the ingredients and preheat the oven at 350° F and grease the 9x13 baking dish or line with a parchment paper, leaving on overhangs on the side, so that you can lift the baked Blondies out and cut easily. Set it aside.
In a large bowl, whisk the melted butter and brown sugar until combined. Whisk in the peanut butter, then add eggs, egg yolk and vanilla extract. Whisk it all until all well combined.
Add the flour, baking powder and salt. Fold the flour into the wet ingredients using rubber spatula. Fold in chopped Resse's mini cups. Batter should be thick. Spread the batter into the baking dish evenly. Sprinkle the Reese's pieces on the top.
Bake for 30 minutes until the toothpick comes out clean or with soft crumbs. Be on the lookout at the end of the time. Make sure not to over bake it otherwise they will be dry.
Remove from the oven and place it on the wire rack to cool completely before you cut them into pieces.
Cover and store blondies at room temperature for 1 week.