1Spray 13x9-inch pan with cooking spray. In 4-quart Dutch oven, heat marshmallows and butter over low heat, stirring constantly, until melted and smooth. Remove from heat.
2Stir 8 cups of the cereal into marshmallow mixture. Press into pan. Cool 30 minutes.
3Spread frosting over marshmallow-cereal layer. Place remaining 1 cup cereal in resealable food-storage plastic bag; seal bag and crush with rolling pin or flat side of meat mallet. Sprinkle crushed cereal and broken candies over frosting.
4Cut into 8 rows by 4 rows. Insert 1 cookie half into each bar to look like tombstone.