Line a baking sheet with parchment and set aside. (Feel free to use foil, if you'd prefer. Just don't forget to grease the foil very well with butter!)
Melt the butter in a large pot, over medium heat. Add the marshmallows and stir until they melt. Remove from the heat and stir in the cereal until well coated. (If desired, add the additional marshmallows now.)
Scoop the sticky cereal mixture onto the prepared pan. Lightly wet your hands (or butter them, if you're like me) and gently press the cereal out across the pan, taking care not to press too hard and compact them. Let cool for a few minutes before slicing. Store in an airtight container for several days. Enjoy!
For an especially gooey treat, you can add additional marshmallows right at the end, just before pressing the mixture into the pan. I like the rice krispy treats both ways, but for the most part, I add those extra marshmallows every time.