Spray an 8x8” baking pan with non-stick cooking spray. In a large pot over low heat, melt butter. Add marshmallows and stir with a rubber spatula until melted. Turn off the heat and fold in cereal. Press cereal and marshmallow mixture into prepared pan*. Let cool while preparing the chocolate.
Place chocolate chips in a microwave safe bowl and microwave on high for thirty seconds, stir. Repeat until all the chocolate has melted. Evenly spread chocolate over prepared bars and let cool until chocolate has set.
Preheat broiler on high. Spread marshmallow fluff over cooled bars and place under broiler for 45 seconds to 1 minute, turning occasionally, until topping has browned. Chill for at least 15 minutes before cutting into bars.
*Tip – Spray you hands with non-stick cooking spray to avoid the marshmallow sticking while pressing the cereal into the pan.