Bring a large saucepan of water to a boil. Add pasta and cook according to package directions, until al dente, 10 to 12 minutes.
Meanwhile, position rack on top shelf in oven and preheat broiler to high. In a medium-size bowl, whisk milk with Dijon, salt and cayenne. Melt butter in a large saucepan set over medium heat. Whisk in flour. Mixture will be dry and very lumpy. Gradually add milk mixture, a little at a time, whisking constantly between additions to beat out any lumps. Stir often until mixture comes to a boil, 3 to 6 minutes. Remove pan from heat and gradually stir in 1-1/2 cups cheese. Continue stirring until melted.
When macaroni is cooked, drain well. Do not rinse. Stir into cheese sauce. Mixture will be very creamy and saucy.
Pour into an 8-inch baking dish. Place on a rimmed baking sheet. Sprinkle with remaining 1/2 cup cheese and then sprinkle breadcrumbs. Broil on top rack of preheated oven until cheese melts, about 5 minutes.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.