Heat the oven to 375°F, grease a 9-by-13-inch baking dish and boil the macaroni 2 minutes short of tender.
Melt 3 tablespoons butter in a saucepan, whisk in the flour and cook for 1 minute.
Whisk in the warm milk and simmer until thickened, then remove from the heat and stir in both cheeses, hot sauce and salt.
Fold the drained macaroni, cooked chicken and celery into the sauce and spread the mixture in the dish.
Mix the panko with the melted butter and scatter it across the top.
Bake until bubbling and browned, 20 to 25 minutes, then rest for 5 minutes.