Line 13x9-inch pan with foil or waxed paper; grease generously with cooking spray. In 3-quart saucepan, melt butter over medium heat. Add marshmallows, stirring until melted. Remove from heat; stir in cereal. Press evenly in pan using greased spatula or waxed paper. Cool completely, about 30 minutes.
Line cookie sheet with waxed paper. Using greased 3-inch tombstone-shaped cookie cutter, cut out 16 tombstones. In small bowl, place crushed wafer cookies.
In small microwavable bowl, microwave 2/3 cup of the frosting on High 20 to 30 seconds or until melted. Dip bottom of each treat into melted frosting, tapping off excess. Immediately coat dipped portion with cookie crumbs. Place on cookie sheet to set, about 10 minutes.
Place vanilla frosting in small resealable food-storage plastic bag; seal bag. Cut off tiny corner of bag. Twist bag above frosting; squeeze bag to pipe RIP (Rest In Peace) on each treat.