Line an 8x8 glass pan with a piece of parchment paper. Set aside.
Measure 3 cups of whole pretzels into a large bag. Crush pretzels until pieces are broken, but not turned to crumbs. Add chopped peanuts to the bag, toss to combine and set aside.
In a large glass bowl, melt butter in the microwave for 45 seconds. Add 1 cup Reese's Minis Peanut Butter Cups and marshmallows and return to microwave for 1 minute 15 seconds.
Stir gently, but quickly until peanut butter cups are mostly melted. Add in pretzel-peanut combination and stir until well coated. As mixture cools it will get slightly more difficult to stir. Transfer to lined pan and quickly press mixture down into pan. Sprinkle Reese's Pieces and remaining 1/2 cup Reese's Mini Peanut Butter Cups on top and gently press into the top of the bars.
Let cool. Lift parchment from pan and cut into small squares. Makes 16-20 squares.