Pulse bread and butter in food processor until crumbs are no larger thatn 1/8 inch, ten to fifteen 1 second pulses.
Bring 4 quarts water to boil in Dutch over over high heat. Add macaroni and 1 Tbsp salt; cook until pasta is tender. Do not undercook. Drain pasta and set aside in colander.
In now-empty Dutch oven, heat butter over medium-high heat until foaming. Add flour, mustard, and cayenne (if using) and whisk well to combine. Continue whisking until mixture becomes fragrant and deepens in color, about 1 minute.
Gradually whisk in milk; bring mixture to boil, whisking constantly (mixture must reach full boil to fully thicken). Reduce heat to medium and simmer, whisking occasionally, until thickened to consistency of heavy cream, about 5 minutes.
Off heat whisk in cheeses and 1 tsp salt until cheeses are fully melted. Add pasta and cook over medium-low heat, stirring constantly, until mixture is steaming and heated through, about 6 minutes.
Either leave in oven proof dutch oven or transfer to 9×13 baking dish and sprinkle evenly with bread crumbs. Broil until crumbs are deep golden brown, 3 to 5 minutes, rotating pan if necessary for even browning. Cool about 5 min., then serve.