01 - Preheat the oven to 400°F. Lightly butter a medium baking dish and set aside.
02 - Bring a large pot of generously salted water to a rolling boil. Add the macaroni and cook according to package directions until al dente. Drain through a colander and set aside.
03 - In a medium saucepan over medium heat, melt 2 tablespoons of butter. Sprinkle in the flour and whisk continuously for 1 to 2 minutes until the mixture is smooth and lightly golden, forming a roux.
04 - Gradually pour in the milk while whisking vigorously to prevent lumps from forming. Continue cooking and stirring over medium heat for 3 to 5 minutes, until the sauce thickens enough to coat the back of a spoon.
05 - Remove the saucepan from heat. Add the grated cheddar, Gruyère, and Dijon mustard, stirring gently until the cheeses are completely melted and the sauce is smooth. Season with salt and pepper to taste.
06 - Add the drained macaroni to the cheese sauce and fold together until every piece is evenly coated. Transfer the mixture to the prepared baking dish, spreading it out in an even layer.
07 - In a small bowl, combine the breadcrumbs with the remaining grated cheddar. Drizzle in the melted butter if using, and toss until the crumbs are evenly moistened. Sprinkle the topping in an even layer over the macaroni.
08 - Place the dish in the center of the oven and bake for 15 to 20 minutes, until the top is deeply golden brown and the sauce is bubbling around the edges.
09 - Remove from the oven and let the dish rest for 5 minutes to allow the sauce to set slightly before serving.