Cook the elbow macaroni according to the package directions for 1 minute less than al dente, then drain immediately
While the pasta is cooking, start on the cheese sauce. Melt the butter in a sauce pan. Add the flour and whisk 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 and 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