Lightly grease a 9 × 13-inch baking pan with butter or line it with parchment paper, leaving some overhang on the sides for easy removal. Measure the cereal and set aside 1 cup of mini marshmallows for folding in later.
Add the butter, chocolate chips, corn syrup, and the remaining marshmallows to a large saucepan. Heat over low heat, stirring constantly until the butter and chocolate melt and the marshmallows become smooth. Avoid overheating the mixture, as it can become overly sticky.
Remove the saucepan from the heat and stir in the vanilla extract and salt. Mix until everything is evenly combined and glossy.
Place the Golden Grahams cereal in a large mixing bowl. Pour the warm chocolate-marshmallow mixture over the cereal and gently fold with a rubber spatula until the cereal is evenly coated.
Add the reserved mini marshmallows and gently mix them through the coated cereal. Working quickly helps keep the marshmallows distinct and prevents the mixture from setting before it reaches the pan.
Transfer the mixture to the prepared baking pan. Lightly grease your spatula or hands and gently press the mixture into an even layer. Avoid packing it too firmly, which can make the bars dense and hard to bite.
Let the bars cool at room temperature for about 1 hour, or until firm enough to cut. Lift them out using the parchment overhang and slice into 12 bars with a sharp, lightly greased knife.
