Start by greasing your baking pan or lining it with parchment paper. In a large saucepan, melt the butter over low heat. Add the chocolate chips, corn syrup, and all but one cup of the mini marshmallows. Stir slowly until everything is smooth and fully combined. Remove the pan from heat and stir in the vanilla extract for an extra layer of flavor.
Pour the Golden Grahams cereal into the saucepan and gently stir until every piece is coated with the chocolate-marshmallow mixture. Then, fold in the reserved cup of mini marshmallows for an extra gooey and chunky texture that resembles roasted s’mores.
Transfer the mixture into your prepared pan and press it down evenly using a buttered spoon or lightly wet hands to prevent sticking. Allow the bars to cool at room temperature for at least an hour, or until they’re completely firm. Once set, cut into squares or rectangles and serve.