Spray bottom and sides of 8-inch (20 cm) square pan with cooking spray.
Place 1 Tbsp. butter and 3 cups marshmallows in large microwaveable bowl.
Microwave on HIGH 45 sec. Remove using pot holders; stir mixture with rubber scraper. Microwave 30 sec. or until marshmallows are melted. (Parents should supervise this step.)
Measure 1/2 cup peanut butter. Remove 2 Tbsp. peanut butter and place in small microwaveable bowl; set aside. Stir remaining peanut butter into marshmallow mixture. Immediately add 4 cups cereal; stir until well coated.
Spray finger tips with cooking spray. Press cereal mixture with finger tips in pan.
Place chocolate squares in small microwaveable bowl with remaining peanut butter. Microwave on HIGH 30 sec. Stir with rubber scraper. Microwave 30 to 45 sec. or until chocolate is completely melted and mixture is smooth when stirred.
Pour chocolate mixture over cereal treats; spread with rubber scraper to cover. Refrigerate 15 min. until frosting is set. Cut into 16 squares.