Double-Chocolate Chewy Crispy Bars

Melt butter and marshmallows over medium-low heat. Stir cocoa into melted marshmallows; cook 3 minutes. Add cereal; toss well to combine. Press cereal mixture into a 13 x 9-inch pan coated with cooking spray. Heat bittersweet chocolate in microwave at HIGH 45 seconds or until melted, stirring every 15 seconds; drizzle over cereal mixture. Chill 10 minutes before slicing.