Cut 18x12-inch sheet of waxed paper; place on work surface. In 4-quart Dutch oven, melt butter and marshmallows over low heat, stirring frequently, until smooth. Stir in cereal. Spread cereal mixture on waxed paper; cool 5 minutes.
Cut 34x12-inch sheet of waxed paper; place on work surface. Lightly spray hands with cooking spray. With hands, shape cereal mixture into about 2 1/2-inch balls.
In small microwavable dish, microwave chocolate chips and shortening on High 1 to 1 1/2 minutes, stirring until smooth. Drizzle chocolate over cereal balls. Let stand at least 30 minutes until chocolate is set before serving.