Prepare a 9 x 13 inch casserole dish by lining it with parchment paper or spraying with cooking spray. Set aside.
In a deep pot over medium heat, melt the butter. Then add marshmallows. Stir continuously with a big wooden spoon until all the marshmallows have lost shape and melted.
Remove from heat and add cereal. Stir well to incorporate. Pour mixture into casserole dish. Use a spoon sprayed with cooking spray to press the mixture firmly down into the dish.
Refrigerate treats for 15-30 minutes or until firm. Cut into squares and serve.