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Line 13x9-inch baking pan with foil. Lightly butter or spray foil with no stick cooking spray. Set aside. Melt butter in large saucepan or Dutch oven on low heat. Add marshmallows; stir constantly until marshmallows are melted. Remove from heat. Stir in vanilla until smooth. Quickly add cereal to marshmallow mixture and stir until well coated.
Press into prepared pan using a buttered spatula. Let stand until cereal mixture is firm. Cut out with 2-inch cookie cutters or cut into bars.
Spoon frosting into medium microwavable bowl. Microwave on HIGH 20 to 30 seconds, stirring every 10 seconds. (Frosting should be a pourable consistency.) Stir in food color.
Dip half of each treat into frosting. Coat with sprinkles, if desired. Place on wax or parchment paper-lined tray. Let stand 20 minutes or until frosting is dried. Turn treats over. Let stand 20 minutes longer or until firm.