Line a 9-inch square baking pan with foil and grease with non-stick cooking spray; set aside.
In a large pot over low heat, melt the butter.
Add the marshmallows and mashed banana and stir constantly until the marshmallows have all melted.
Remove the pot from the stove and stir in the cereal until evenly coated with the marshmallow cream. Turn the mixture into the prepared pan and press it into a compact, even layer. Refrigerate until set, about 30 minutes. Store leftovers in an airtight container in the refrigerator for up to 4 days.