Boil the macaroni in 3 quarts water with 1 tbsp salt until just tender, 1 minute less than the package directs, then drain without rinsing.
Melt the butter in the empty pot over medium heat, whisk in the flour and cook until foamy but not browned, 1 minute.
Gradually whisk in the warm milk and cook until the sauce lightly coats a spoon, 4 to 5 minutes.
Stir in the dry mustard, 1/2 tsp salt and pepper and lower the heat to its minimum setting.
Add the cheddar and American cheese by handfuls, stirring each addition smooth before adding the next; do not let the sauce boil.
Fold in the macaroni and cook for 1 minute until the sauce clings while still looking a little loose.
Serve immediately, loosening with 2 tbsp warm milk if the mac waits more than five minutes.
