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.
Notes
***This recipe is best if made and devoured the same day. Otherwise the cereal gets soft the next day. I believe this is due to the high content of corn syrup in the recipe. I have had readers freeze it until ready to eat and it hasn't been soggy.