Coat a 13x9 pan with canola oil spray and set aside. Remove cereal bag from outer box. With cereal still in the bag, use your hands to crush cereal into smaller pieces; measure out 6 cups and set aside.
In a large pan, melt butter over medium heat. Add marshmallows and stir until melted. Turn off heat and add crushed cereal and 2/3 of the peanut butter cups to pan. Stir quickly to incorporate into marshmallow mixture.
Press the cereal mixture into greased pan with the back of a spoon. Top with remaining peanut butter cups. Using a piece of waxed paper, gently press peanut butter cups into treats. Let cool, then cut into squares and enjoy!