01 - Preheat the oven to 400°F (200°C). 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 just al dente. Drain thoroughly in 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 turns a pale golden color and develops a nutty aroma.
04 - Gradually pour in the whole milk while whisking vigorously to prevent lumps from forming. Continue cooking and stirring steadily 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 all the cheese has melted and the sauce is smooth. Season with salt and pepper to taste.
06 - Add the drained macaroni to the cheese sauce and fold 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 and remaining grated cheddar. Drizzle with melted butter if desired and toss to combine. Scatter the topping evenly over the entire surface of the macaroni.
08 - Bake on the center rack for 15 to 20 minutes until the topping 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.