In a large saucepan, melt the butter over low heat. Add the marshmallows and stir until completely melted and smooth.
Remove the saucepan from the heat and stir in the chocolate chips and peanut butter (if using) until melted and well combined.
Gently fold in the Rice Krispies cereal until evenly coated with the chocolate marshmallow mixture.
Let the mixture cool slightly. Then, using your hands or a spoon, form the mixture into balls.
Place the balls on a parchment-lined baking sheet and let them set for at least 30 minutes before serving.