The trick is in the proportions of crisp topping, creamy sauce, and smooth pasta—and Chef Rosen gets it exactly right. She advises that you have some freedom in choosing your pan: just remember that a large, flat pan will give you a greater area of crumb topping than a small, deep one.
6 tablespoons butter, plus extra for the baking dish
¼ cup corn bread crumbs (or more, depending upon pan size)
1 pound elbow macaroni
1 quart whole milk
6 tablespoons flour
Kosher salt
Freshly ground pepper
1¼ cups (5 ounces) grated Gruyère cheese
1¼ cups (5 ounces) grated mild white Cheddar cheese
1¼ cups (5 ounces) grated Grana Padano or Parmesan cheese
Preheat the oven to 350°F. Lightly butter a 3½-quart deep baking dish or a 9 by 13-inch baking pan.
Spread the crumbs in a single layer on a baking sheet and bake until golden brown, 10 to 15 minutes. Set aside.
Bring a large pot of salted water to a boil and cook the macaroni until al dente, 8 to 10 minutes. Drain and place in a large bowl. (To prepare up to a day ahead, mix in a small amount of canola oil, cover with plastic wrap, and refrigerate until ready to use.)
Bring the milk to a simmer in a small saucepan over medium-high heat.
In a medium saucepan over medium-low heat, melt the 6 tablespoons of butter, add the flour, and mix well with a wooden spoon or spatula. Cook, stirring, for 3 minutes. Whisk in the hot milk and continue whisking until smooth. Raise the heat to medium and cook, stirring continuously, until the mixture thickens enough to coat the spoon. Season with salt and pepper and strain through a fine strainer.
Add the sauce to the cooked macaroni. Add 1 cup each of the Gruyère, Cheddar, and Grana Padano and mix well. Taste, and season with salt and pepper if necessary.
Pour the macaroni mixture into the baking dish and sprinkle with the remaining cheese. (At this point, the macaroni and cheese may be cooled on the counter, covered, and refrigerated for one day. Before proceeding, preheat the oven to 350°F.)
Sprinkle the toasted corn bread crumbs evenly over the casserole and cover with foil. Bake on the middle shelf until heated through, about 20 minutes; remove the foil and continue baking until the top is golden brown, an additional 10 minutes. Allow to stand for 10 minutes before serving.