The recipe for these chewy treats comes from Sonny Lewis, Bethesda, Maryland.
Coat a 9-by-13-inch baking dish with cooking spray. Line with wax or parchment paper, leaving an overhang on two sides; coat paper with cooking spray.
Combine marshmallows, butter, peanut butter, and corn syrup in a large (6-quart) pot; cook over medium heat, stirring, until melted and smooth, 5 to 8 minutes. Remove from heat. Add cereal; stir to coat.
Quickly transfer mixture to prepared dish, sprinkling in chocolate chunks as you go. Using a metal spatula coated with cooking spray, press in firmly. Refrigerate until chocolate is set, 10 to 15 minutes. Run a knife around edges; lift out of baking dish, using paper as an aid. Cut into bars.
Work quickly, and transfer the cereal mixture to the baking dish while it is still warm, so it is easy to distribute the chocolate chunks evenly.
You can substitute white-chocolate chips, raisins, or chopped nuts for the semisweet chocolate.