Microwave butter in a large microwavable bowl for 45 seconds on high until melted.

Add the marshmallows and toss to coat.

Microwave 1 1/2 minutes, stirring every 45 seconds, until marshmallows are completely melted and the mixture is blended well.

Add cereal and mix well. Press into a greased 9X13 pan and cool completely.

Cut into squares.