Line a 9x9 square baking pan with parchment paper and spray with non-stick cooking spray. If wanting thinner squares, use a 9x13 inch pan.
In a large pan, melt butter and peanut butter on low heat, stirring constantly.
Once butters have melted add mini marshmallows. Stir constantly to avoid burning.
When marshmallow mixture has melted and is smooth, turn off heat and remove pan from stove.
Add cereal to pan and stir until evenly coated with mixture.