Spray a shallow baking dish with nonstick cooking spray, or lay out a large piece of parchment paper.
In a large pot or stock pot, heat the butter over medium heat.
Once butter is fully melted, add the marshmallows, and stir. Keep stirring to get marshmallow nice and creamy.
Once completely melted, remove from heat and slowly add the cereal to the melted marshmallow folding until all the cereal is coated in marshmallow. (You'll want to do this fairly quickly before the marshmallow mixture gets too cold)
Spread the mixture with a flat baking spatula until its in an even layer. This can be in the shallow baking dish or on the parchment paper. If needed, spray the spatula with cooking spray to keep the mixture from sticking.
Use a fun shaped cookie cutter to make shapes. You can reshape and flatten the cereal treats as needed to cut additional shapes.