1Line jelly roll pan, 15 1/2x10 1/2x1 inch, with aluminum foil. Spray foil with cooking spray.
2Place butter in large microwavable bowl. Microwave uncovered on High about 30 seconds or until melted. Stir in marshmallows. Microwave on High 1 to 3 minutes, stirring every 30 seconds, until marshmallows are melted and smooth. Stir in cereal. Spoon cereal mixture into pan; spread evenly and firmly. Freeze 15 minutes.
3Turn pan upside down onto cutting board; remove foil. Cut cereal mixture into 18 squares. Spoon about 1/4 cup ice cream onto 9 of the squares; spread slightly. Place 4 banana slices on each. Top with remaining squares; press slightly. Freeze about 45 minutes or until firm.
4Cut each sandwich crosswise in half. Wrap each in waxed paper. Freeze up to 1 week.