Spray a 9x13x2-inch baking pan with cooking spray. Line the pan with parchment paper (the parchment should line the bottom and come up and overhang the two longer sides - this will make is easier to remove the treats from the pan after they cool). If you don't have parchment, wax paper will work or you could just use the sprayed pan.
In a large pot (I use a 7-quart pot), melt the butter over low heat.
Set aside ½ cup of the marshmallows and ¼ cup of the chocolate chips (these will be used to top the treats later). When the butter has melted, add the remaining marshmallows and chocolate chips to the pot. Continue cooking over low heat, stirring occasionally, until completely melted and well-combined.
Remove the pot from the heat and stir in the vanilla extract.
Add the Golden Grahams and stir everything together until the cereal is evenly coated with the marshmallow mixture. It's okay if there are a few bare spots here and there, but the cereal should be mostly coated so that the bars do not fall apart. Work quickly here, as the mixture gets harder to work with the more it cools down.
Pour the cereal mixture into the prepared pan. Spread it out evenly and press it down into the pan. I use my fingertips, sprayed a little with cooking spray, to do this (I think it's the easiest way to get the job done), but a spoon or spatula sprayed with cooking spray works too.
Sprinkle the remaining marshmallows and chocolate chips evenly over the top and gently press them down into the treats so that they stick.
Cool the treats until firm, about an hour.
Cut the treats into 24 squares.