Grease an 8-inch or 9-inch square pan with butter or nonstick baking spray.
Grease the inside of a large mixing bowl with butter or non-stick baking spray then pour in the Frosted Flakes cereal.
Begin to melt 4 tablespoons butter in a medium saucepan set over low heat.
Then stir in 10 ounces of marshmallows and allow them to melt, stirring often, until about 75% have melted.
Remove the marshmallows from the heat and stir, smashing them, until they are completely melted.
Pour the marshmallow mixture over the cereal and toss to coat evenly.
Spread the cereal mixture into the prepared pan then set aside for at least 1 hour in order for it to cool before cutting into 16 squares.
Store the treats in an airtight container for up to 1 week.
Optional Microwave Method:
Heat the butter and marshmallows on high for one minute then remove and stir until melted and smooth.
Heat for an additional 15-second burst of high power, only if needed. Pour over the cereal and continue with the recipe listed above.
Heating the marshmallows just until about 3/4 of them melts, then stirring until melted off-heat, keeps your marshmallows from overheating which causes the sugar to caramelize and makes your treats too hard.