These no-bake bars come together quickly with common pantry ingredients. Make sure the cereal is well crushed (try packing it in a sealed zip-top plastic bag and using a rolling pin) so it incorporates into the peanut butter mixture.
Combine first 3 ingredients in a heavy saucepan over medium-high heat. Cook 4 minutes or just until mixture begins to boil, stirring constantly. Remove from heat; stir in peanuts and cereal. Spread mixture evenly into a 13 x 9–inch baking pan coated with cooking spray.
Place the dark chocolate in a small microwave-safe bowl. Microwave at HIGH 1 minute or until chocolate melts, stirring every 20 seconds. Drizzle chocolate evenly over peanut mixture. Score into 36 bars while warm.