Night Owl S'mores Treat

Grease a cookie sheet with butter. Measure 9 cups of graham cereal into a large mixing bowl. In a medium saucepan over medium heat, combine corn syrup, butter and milk chocolate chips. Stir constantly until chocolate chips are melted and ingredients are combined. Remove from heat and stir in vanilla. Pour over cereal and stir quickly until cereal is coated with chocolate. Stir in the marshmallows. Pour mixture onto cookie sheet and using a slightly damp hand, press into the pan.