1. Line 13x9-inch pan with foil. Lightly butter or spray foil with no stick cooking spray. Set aside.
2. Melt butter in large saucepan or Dutch oven on low heat. Add marshmallows; stir constantly until marshmallows are melted. Remove from heat.
3. Stir in vanilla and food color until smooth. Quickly add cereal to marshmallow mixture and stir until well coated.
4. Press into prepared pan using a buttered spatula. Cool. Cut into bars or use a shamrock-shaped cookie cutter.