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 a smooth, golden roux forms.
04 - Gradually pour in the whole milk while whisking constantly to prevent lumps from forming. Continue cooking and stirring 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 until the cheeses are fully melted and the sauce is smooth. Season with salt and black pepper to taste.
06 - Fold the drained macaroni into the cheese sauce, stirring gently until every piece is evenly coated. Transfer the mixture to the prepared baking dish, spreading it into 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 mixture resembles wet sand. Sprinkle evenly over the macaroni.
08 - Bake on the center rack for 15 to 20 minutes until the topping is golden brown and the edges are bubbling.
09 - Remove from the oven and let the dish rest for 5 minutes before serving to allow the sauce to set slightly.