01 - Preheat your oven to 400°F. Grease a 9x13-inch baking dish.
02 - Cook the elbow macaroni according to package directions until al dente. Drain thoroughly and set aside.
03 - In a large saucepan, melt 2 tablespoons of butter over medium heat. Whisk in the flour and cook for 1 minute, stirring constantly.
04 - Gradually whisk in the 2 cups of whole milk, stirring continuously until the mixture is smooth and has thickened, approximately 3-4 minutes.
05 - Reduce the heat to low. Add the shredded cheddar and mozzarella cheeses, kosher salt, and black pepper. Stir until the cheeses are fully melted and incorporated, creating a smooth sauce.
06 - Gently fold the cooked macaroni into the cheese sauce until it is evenly coated. Pour the mac and cheese mixture into the prepared baking dish.
07 - Evenly sprinkle the semisweet chocolate chips over the surface of the mac and cheese. Distribute the mini marshmallows on top of the chocolate chips.
08 - In a small bowl, combine the crushed graham cracker crumbs, melted butter, brown sugar, and a pinch of salt. Mix until the ingredients are well combined and the mixture is crumbly. Sprinkle this topping evenly over the marshmallows.
09 - Bake the dish uncovered in the preheated oven for 10-12 minutes, or until the marshmallows are golden brown and the graham cracker topping is crisp.
10 - Allow the dish to rest for 5 minutes before serving to allow the flavors to meld and the dish to set slightly.