Pour cereal into an extra-large mixing bowl; set aside. Butter a 9x13 pan; set aside.

Melt 6 tablespoons butter in a large saucepan over low heat. Add marshmallows and stir constantly until completely melted. Remove from heat.

Working quickly, pour marshmallow mixture over cereal, gently stirring until the cereal is well coated. (Don't over-mix or the marshmallow treats will start melting.)

When the treats are cooled, cut them into 12 bars with a buttered knife, and wrap each one with plastic wrap. Best if served the same day.