1. Put butter, marshmallows and peanut butter pieces into a 2-quart glass casserole. Cook uncovered in microwave oven 1 1/2 to 2 minutes at BAKE, or until melted and smooth; stir once.
2. Mix cereal into marshmallow mixture and press into a lightly buttered 1 1/2-quart baking dish. Cool until set. Cut into bars.