Place marshmallows and butter into a large bowl and microwave until puffed up, about 3 minutes.
Remove the marshmallow mixture from the microwave and pour into cereal. Mix to combine, then pour into a greased 9×13 inch baking dish and press to make even.
Melt peanut butter until nice and runny, about 45 seconds. Pour in quartered peanut butter cups, stir gently then pour over treats.
Drizzle melted chocolate over top and refrigerate to harden. When set, cut into squares and serve!