In a large microwave-safe bowl, microwave butter, 4½ cups of marshmallows, and the chocolate chips on HIGH for 2 minutes, stirring halfway through, until melted.
Stir cereal into the chocolate marshmallow mixture until well coated. Stir in the remaining 1½ cups of marshmallows, then press into the prepared pan.
Optional: Top with additional marshmallows and a drizzle of melted chocolate (or more chocolate chips).
Cool completely before cutting into bars.
Notes
Keep leftovers in an airtight container in the refrigerator for 7 days and in the freezer for 4 months.