Grease a 9-inch x 13-inch baking dish and wooden spoon. Set aside.
Place a large pot over medium heat. Melt butter and 6 cups mini marshmallows together using your prepared spoon to stir things up.
Add the light corn syrup and stir to combine.
Add the chocolate chips and cook until melted, stirring constantly.
Once the chocolate is melted, pour in the Golden Grahams cereal. Stir until cereal is coated in marshmallow/chocolate mixture.
Pour cereal into your prepared pan and press down into an even layer. (I like to spray some non-stick cooking spray on my hands to keep things from sticking to them.)
Spread the remaining 6 cups mini marshmallows on top of the treats in an even layer.
Move oven rack to top slot and preheat the oven to broil.
Once preheated, place pan into the oven. Toast until marshmallows are golden brown. This happens very quickly, so DO NOT walk away!
Carefully remove from oven. Cut into 12 bars immediately, or let rest 5 to 10 minutes before cutting for cleaner edges.
Notes
I will spray my clean hands and use them to spread the Golden Grahams into even layer. Be very careful though as they are hot.