Grease a 13x9-inch baking pan with cooking spray or butter.
Melt the butter in a large pot over low heat. Once the butter is melted, add the marshmallows and cook until the marshmallows have melted, stirring occasionally.
Stir in the vanilla extract.
Remove the mixture from the heat and stir in the peanut butter and Cheerios until everything is coated evenly.
Transfer the mixture to the baking pan, pressing it into one even layer.
Let the bars cool, then use a knife to cut into squares.
You can eat then immediately or store them in an airtight container at room temperature.
Recipe by Ally's Cooking at https://allyscooking.com/2017/09/13/peanut-butter-marshmallow-cheerios-bars/