Grease a 13x9 baking pan with butter or a cooking spray and set aside.
In a large pot, heat marshmallows and butter over low-medium heat. Stir consistently.
Once melted, remove from heat. Stir in your Rice Krispie cereal, mixing well and fast until the cereal is evenly coated. Add 1 cup of the mini marshmallows, ¾ cup of broken graham crackers, and ½ cup of chocolate chunks. Mix well.
Press the mixture into the greased pan using a spatula.
Sprinkle the top with the remaining mini marshmallows, graham crackers, and chocolate chunks. Use a spatula to help press in the toppings.
Let cool for 1 hour before cutting or consuming.