Bring pasta water to a boil in a small saucepan, add salt and macaroni and cook for 7 to 9 minutes until tender. Drain well.
Melt sauce butter in the empty pan, whisk in flour and cook for 30 seconds. Gradually whisk in milk and simmer for 1 minute until thickened, then switch off the heat and stir in cheddar.
Fold macaroni into the thick sauce and spread on a plate for 5 minutes to let steam escape; keep it warm rather than chilling it.
Divide half the American cheese over 2 brioche slices, add the warm macaroni evenly in flat layers and top with remaining American and bread. Butter the outside faces.
Cook in a large skillet over medium-low heat for 4 to 5 minutes per side, covering for 2 minutes after turning, until golden and the macaroni filling reaches 165°F.
Rest for 2 minutes, then cut carefully and serve one sandwich per person.