Spray a 9x13 pan with nonstick cooking spray and set aside. Also, spray a large pot and a rubber scraper with nonstick cooking spray.
Place pot over medium heat and melt butter. Pour in 10 oz (one bag) of mini marshmallows and stir to coat in butter. Continue stirring until marshmallows have melted completely. Remove from heat.
Pour in cereal and stir to coat in melted marshmallow. Once mostly coated, stir in the remaining 5 oz. marshmallows. They will melt slightly from the heat, but not completely which is what you want. Pour mixture into prepared 9x13 pan.
Using lightly greased fingers, press bars down to flatten the top. Cool 15-30 minutes before cutting into squares.
Store in an airtight container for up to 3 days to maintain freshness.