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