Line an 8 or 9-inch baking pan with non-stick foil or lightly greased parchment and set aside.
Spray a large bowl with cooking spray, add the cereal, and set aside.
In a microwave-safe bowl, place the marshmallows and butter. Microwave for 1 minute. Stir the mixture very well, until smooth.
Pour the marshmallow mixture over the cereal and stir very well with a spatula that has been greased with cooking spray.
Transfer to the prepared baking pan and press evenly with a spatula.
Let cool 2 hours. You may refrigerate to speed things up.
Cut into 25 squares for small treats (or 16 for larger).
Notes
To make on the stove-top, place the butter and marshmallows in a saucepan over medium-low heat. Heat, stirring, until 75% of the marshmallows are melted. Remove from heat and use the residual heat to fully melt the marshmallows. The mixture should be very smooth. TIP: Spray bowls and spatulas with non-stick cooking spray for easy treats. For a larger batch in a 9 x 13 inch baking pan, use these amounts:
6 tablespoons salted butter
16 ounces of marshmallows (just over 9 cups of mini marshmallows OR about 68 regular marshmallows)