Preheat oven to 375°F.
In a large saucepan, or saucier, over medium-low heat, melt butter.
When the butter begins to sizzle slightly, add flour, dry mustard, and salt. Whisk with the butter to form a light roux.
Whisk in milk one cup at a time; continue whisking until the sauce has thickened slightly.
Whisk in 12 ounces of shredded cheddar cheese, one handful at a time, whisking until the cheese is melted after each addition. Continue whisking until all the cheese is melted and the sauce has thickened. Reduce heat to low and whisk occasionally, making sure sauce does not burn.
Cook pasta according to al dente package directions. Drain, do not rinse, and return to pot.
Pour hot cheese sauce over pasta and stir to combine.
Spray a large, 9x13, casserole dish with non-stick cooking spray.
Pour macaroni and cheese into the prepared pan and top with the additional 4 ounces of shredded cheese.
Bake in the 375°F preheated oven for 30 minutes, until golden and bubbly.
Remove from the oven and let sit for 5-10 minutes before serving.
Yield 8 servings.