In a large bowl, pour in box of cereal and set aside

Add butter and all but 1 cup of the marshmallows to a large pot over medium heat. Continuously stirring until all the marshmallows and butter are melted together. Pour in sweetened condensed milk and stir until incorporated

Remove marshmallow mixture from heat and pour directly over cereal in the large bowl. Gently folding the mixture to coat the cereal. Add 3/4th cup of remaining marshmallows to the mixture and stir in. In a parchment lined 9x9 baking pan, spoon in the mixture. Using a clean measuring cup, spray the bottom with cooking spray and lightly press down on top of the cereal treat to compact it into the pan. Top with remaining 1/4th cup of marshmallows

Let sit for at least an hour or set in refrigerator for 30 minutes before cutting. I like to cut the cereal treats and store them in an airtight container in the refrigerator to keep them slightly chilled