Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with non-stick cooking spray or line it with parchment paper. In a small bowl, combine the graham cracker crumbs and melted butter, then press the mixture firmly into the bottom of the prepared dish. Pour the sweetened condensed milk evenly over the crust.

Sprinkle half of the mini marshmallows, all of the chocolate chips, and the broken graham cracker pieces over the top. Bake for 15 minutes. Remove the dish from the oven and evenly scatter the remaining marshmallows on top. Return to the oven and bake for another 10–15 minutes, or until the marshmallows are golden brown and the edges are nicely browned.

Take the bars out of the oven and place the broken chocolate bar pieces over the hot surface. Let them sit for a few minutes to melt slightly. Transfer to a cooling rack and allow to cool for at least 15 minutes before slicing and serving. The bars will be gooey but should hold together. For the best texture and ease of cutting, let them cool completely before serving.