Add the butter to a large pan and begin to heat it on low heat. While the butter heats, spray a 9 x 13 pan with non-stick spray.
Add the peanut butter and marshmallows to the butter on low heat while continually stirring to avoid burning the mixture. Stir the peanut butter, marshmallows, and butter constantly so that it becomes evenly melted.
You’ll want to keep stirring the butter, marshmallows, and peanut butter mixture on low until it is completely melted and mixed together. You may be tempted to turn up the heat to make the process go faster, but don’t. The key is to keep stirring while it cooks on low.
Add the cereal to the mixture and then pour everything into the 9 x 13 pan. Use a spatula to spread the mixture out evenly and let cool for an hour before cutting into squares.