Grease a 13×9 pan with butter.
Reserve 1 c of the marshmallows.
In 3 qt. saucepan, melt chocolate chips, butter, corn syrup and remaining 4 ½ c marshmallows over low heat, stirring occasionally, until completely melted. Remove from heat; stir in vanilla.
Pour cereal into a large bowl. Pour marshmallow mixture over cereal; stir until evenly coated.
Stir in remaining 1 c marshmallows.
With the buttered back of the spoon, press the mixture into the pan.
Cool at least 1 hour or until firm.
Store loosely covered at room temperature.
