Prepare 9x13 baking pan with parchment paper and non-stick cooking spray. Set aside.
In large non-stick pot on medium-high heat, melt butter. Reduce heat to medium and add marshmallows and stir until mostly melted. Reduce heat to medium-low and add peanut butter and stir to combine. It's okay if some marshmallows are still whole.
Remove from heat and stir until most of the marshmallows melt. Add Cheerios and stir until completely coated. Pour into prepared pan and spread evenly. Optional: use a piece of parchment to press Cheerios into an even layer.
Let cool completely, then remove from pan. Optional: Drizzle with melted chocolate. Cut into 24 pieces.