Chocolate-Butterscotch Chewy Crispy Bars

Melt butter and marshmallows over medium-low heat. Add cereal; toss well to combine. Press cereal mixture into a 13 x 9-inch pan coated with cooking spray. Microwave chocolate chips for 1 minute, stirring every 20 seconds. Spread over cereal mixture. Microwave butterscotch chips and milk 30 seconds; stir once. Dollop butterscotch mixture over chocolate; swirl with a knife. Chill.