1Grease 13x9-inch pan with butter. Reserve 1 cup of the marshmallows. In 3-quart saucepan, melt chocolate chips, butter, corn syrup and remaining 4 1/2 cups marshmallows over low heat, stirring occasionally, until completely melted. Remove from heat; stir in vanilla.
2Pour cereal into large bowl. Pour marshmallow mixture over cereal; stir until evenly coated. Stir in remaining 1 cup marshmallows.
3With buttered back of spoon, press mixture in pan. Cool at least 1 hour or until firm. Store loosely covered at room temperature. For bars, cut into 6 rows by 4 rows.