Lightly spray a 9"x13" pan with nonstick spray; set aside.
In a large pot over medium-low heat melt 5 tablespoons of butter and the peanut butter. Stir constantly until melted. Increase the heat to medium, and add the mini marshmallows. Stir constantly until the marshmallows are melted.
Turn the heat off and add the Rice Krispies. Stir until the cereal is evenly coated.
Pour half of the mixture into the prepared pan, and spread into an even layer. Place the REESE'S Peanut Butter Cups on top, and layer with the remaining Rice Krispie mixture.
Add the chocolate chips and 3 tablespoons of butter to a small bowl. Melt in the microwave at 50% power for 30 seconds. Stir and repeat until melted.
Spread the chocolate over the treats, and sprinkle with REESE'S PIECES.
Let the chocolate set and the treat cool, about 1 hour. Slice and serve!
Notes
Leftovers will keep in an airtight container for up to 3 days before they start to harden.