Line a 9x13 inch baking dish with parchment paper and lightly grease with cooking spray. Place the Golden Grahams cereal into a very large mixing bowl.

In a large saucepan over low heat, melt the butter. Add the 10-ounce bag of marshmallows and stir constantly until completely melted and smooth.

Immediately remove the pan from heat. Stir in the vanilla extract and 1 cup of the milk chocolate chips, stirring until the chocolate is mostly melted.

Pour the hot marshmallow-chocolate mixture over the cereal in the large bowl. Using a greased spatula, quickly but gently fold until all the cereal is coated.

Fold in the remaining 1/2 cup of milk chocolate chips and the reserved 1 cup of mini marshmallows so they remain intact.

Transfer the mixture into the prepared baking dish. Using lightly greased hands or a spatula, gently press the mixture evenly into the pan. Do not pack too tightly.

Allow the bars to set at room temperature for 1 to 2 hours before using the parchment paper to lift them out and slice into squares.