Adjust oven rack to upper-middle position and heat oven to 375º. Line a 13- by 9-inch baking dish with parchment, leaving an overhang on the long sides of the dish. Lightly mist the parchment and sides of the dish with cooking spray. Line the bottom with graham crackers, cutting as necessary to cover the bottom of the dish.

Combine 6 tablespoons of butter and the sugar in a large saucepan set over medium heat. When the butter melts and begins to bubble, pour it over the graham crackers and spread evenly. Bake for 7-10 minutes, or until the sugar bubbles. Remove from oven and top with chocolate squares. Return to oven for 2 additional minutes to melt the chocolate. Remove from oven and use an offset spatula to spread the chocolate evenly over the graham crackers.

Return saucepan to burner and melt the remaining 2 tablespoons of butter over medium-high heat. Add the marshmallows and cook, stirring frequently, until the marshmallows are completely melted. Add the cereal and stir until incorporated. Scape the cereal over the chocolate covered grahams. Lightly grease your hands with cooking spray, then press the cereal mixture into the baking dish. Sprinkle remaining marshmallows and chocolate over the top, gently pressing onto the warm cereal to adhere. Refrigerate to set.

When ready to serve, use the parchment sling to lift the bars from the dish. Cut with a greased knife. Store any leftovers in an airtight container.