Line a 9- by 9-inch baking pan with parchment paper or rub a teaspoon of butter on the bottom and sides to keep the marshmallow crispy treats from sticking to the inside surfaces. Set aside.
In medium or large saucepan, melt the butter over low heat. Pour in the mini marshmallows and stir until melted. Remove from the heat. Add the peanut butter to the melted marshmallows and stir until both ingredients are incorporated. Quickly pour in the puffed rice cereal. Using a spatula, fold the puffed rice cereal into the marshmallow mixture until most of the cereal is coated.
Scrape the mixture onto the prepared baking pan and let cool for 10 to 15 minutes. Once the marshmallow has cooled and hardened, remove from the baking pan and cut into squares or another desired shape. Serve. These peanut butter marshmallow crispy treats can be stored in an airtight container for up to a week.
After using your saucepan, soak it in the sink immediately to keep marshmallows from forming a hard-to-clean crust.