1. Melt butter in a large saucepan over medium heat. Add marshmallows and peanut butter; cook 3 minutes or until smooth, stirring constantly. Remove mixture from heat; stir in cereal and next 4 ingredients (through salt).
2. Working quickly, spread mixture in an 8-inch square metal baking pan coated with cooking spray. Using a wet spoon, spread mixture into an even 1-inch layer; let stand 1 hour or until set. Cut into 16 squares.