Cook the elbow macaroni according to the package directions for 1 minute less than al dente, then drain them immediately.
While the pasta is cooking, start on the cheese sauce. Melt the butter in a sauce pan. Add the flour and whisk for 1-2 minutes constantly, until browned.
Pour the milk in gradually, whisking until smooth.
Continue whisking until the cheese sauce thickens (7-8 minutes).
Remove the sauce from the heat. Stir in the salt and 3 cups of the cheese until smooth.
Mix the cooked macaroni noodles into the cheese sauce.
Grease a 9 by 13 baking dish.
Pour half of the mac and cheese in and spread it out.
Top with 1 cup of the shredded cheese.
Add the last half of the pasta.
Top with the remaining shredded cheese.
Stir together the ingredients to make the panko topping: bread crumbs, melted butter, and salt.
Top the macaroni and cheese with the panko bread crumb mixture.
Bake the macaroni and cheese at 375 degrees for 15-20 minutes, until golden brown and bubbly.