Spray 13x9-inch (3-quart) glass baking dish with cooking spray. In large saucepan, melt butter over low heat. Add marshmallows; cook, stirring constantly, 4 to 5 minutes or until melted and smooth. Remove from heat. Stir in cereal and cranberries until well coated.
Press mixture into baking dish. Let stand 10 to 15 minutes or until firm. Cut into 6 rows by 4 rows.