Measure out 7 cups of cereal and place in large bowl. Set aside. Prepare a 13x9 dish by spraying with nonstick spray. Set aside.
Add butter to a medium/large saucepan. Melt over medium heat. Once melted, add marshmallows and stir continuously until the marshmallows turn gooey and liquefied, about 5 minutes. Remove from heat.
Add melted marshmallows directly over the cereal. Quickly toss together to evenly coat the cereal. Place mixture into prepare 13x9 pan and press down with wooden spoon or spatula. Quickly sprinkle on remaining marshmallows and Hershey bar pieces and press into mixture to help them stick.
Preheat broiler on low and place the 13x9 pan with the cereal bars under the broiler for 1 minute, just to melt the chocolate. Remove when chocolate has melted and not a second after!
If you don't mind messy bars, you may enjoy immediately. But if you want clean-cut bars, let them cool in the fridge for at least one hour.