In a large pot, melt the butter over low heat. Once melted, add the marshmallows and stir constantly until they have all melted. Remove from the heat and stir in the cereal until it is completely coated with the melted marshmallow sauce.
Turn the mixture out into the pan and, using a spoon or spatula that has been sprayed with non-stick cooking spray, press into an even layer. Let sit at room temperature for at least 30 minutes, then cut into squares and serve. Leftovers should be stored in an airtight container at room temperature for up to 3 days.