In a large microwave-safe bowl (or pan on the stove over low heat), melt butter & marshmallows. Stir often and make sure not to overheat or burn.
In the meantime, place graham crackers into a plastic bag and smash with your hand or a rolling pin.
Once marshmallows are melted, remove from heat. Stir in ⅓ cup crushed graham crackers. You want to work quickly here as the mixture stiffens up as it cools, making it hard to stir.
Dump in the crispy rice cereal. Start with 7 cups of rice cereal. I like ooey gooey treats. However, if you like a firmer treat, add 8 cups or more to your liking.
Stir in ⅔ cup crushed graham crackers. Stir again. There should be a few graham cracker crumbs left for the top.
Spray your 8x8 pan or 9x9 cake pan with oil. Pour the crispy treat mixture into the pan. If needed, grease your hands to pat down the treats into your pan.
Break your chocolate bar into rectangles. Press chocolate & remaining graham cracker crumbs into the top of your treats.
Allow to sit and cool. Or, dive right in for a melty, ooey gooey S'more Crispy Treat!
Cover with foil or store in an airtight container until you devour them.