Boil the macaroni in salted water for 2 minutes less than the package directs, then drain it.
Melt 3 tablespoons butter in a large oven-safe skillet, whisk in the flour and cook for 1 minute.
Slowly whisk in the warm milk, mustard, salt and pepper and simmer until the sauce coats a spoon, 4 to 5 minutes.
Remove the skillet from the heat and stir in the cheddar by handfuls until smooth.
Fold in the macaroni, then spoon the pulled pork and barbecue sauce in broad ribbons across the top and turn them through only twice.
Melt the remaining butter, toss it with the panko and scatter the crumbs over the skillet.
Broil 2 to 3 minutes until the crumbs brown, watching continuously, and rest for 5 minutes before serving.
