Pasta:
Cook as directed on package. NOTE: It's crucial to cook the pasta until tender--just past the "al dente" stage. Once prepared, set aside until sauce is completed.
Adjust oven rack to lower-middle position and heat broiler.
Bread Crumb Topping:
Pulse bread and butter in food processor until crumbs are no larger than 1/8 inch, ten to fifteen 1-second pulses. Once completed, set aside until pasta and sauce are mixed.
Cheese sauce:
1. Heat butter over medium-high heat until foaming.
2. Add flour and whisk well to combine. Continue whisking until mixture becomes fragrant and deepens in color, about 1 minute.
3. Gradually whisk in milk; and bring mixture to boil, whisking constantly. The mixture must reach full boil to fully thicken.
4. Reduce heat to medium and simmer, whisking occasionally, until thickened to consistency of heavy cream, about 5 minutes.
5. Remove from heat, whisk in cheeses and 1 teaspoon salt until cheeses are fully melted.
Combine pasta with cheese sauce
1. Add pasta and cook over medium-low heat, stirring constantly, until mixture is steaming and heated through, about 6 minutes.
2. Transfer mixture to broiler-safe 9-by 13-inch baking dish.
3. Sprinkle evenly with the bread crumbs.
4. Broil until crumbs are deep golden brown, 3 to 5 minutes, rotating pan if necessary for even browning.
5. Cool about 5 minutes, then serve.
