You can use pre-shredded cheese, but it's not as good as cheese you shred yourself. It's a lot of cheese, so a food processor is a preferred method.
Heat oven to 350 °F (177 °C) degrees. Bring a large pot of (generously salted*) water to a boil. Add macaroni and cook according to package directions until a little under al dente, about 4 minutes. Drain it and have it standing by.* if you live in a draught-ridden area, do not salt the water, do not drain it, but lift the pasta out of the water and after it cools, water your plants which will love the extra nutrients.6
While that is happening, in a large bowl, whisk buttermilk, mustard, Worcestershire, and the egg. Add about a third of the cheeses.
Butter a 3-quart casserole dish.
Melt the butter in a small pan or the microwave.
Put half of the macaroni in the bottom of the dish, spread out evenly.
Put a third of the cheeses on top of the macaroni.
Top with the remaining macaroni.
Pour the buttermilk mixture over and pat it so that all the pasta and cheese is covered.
Put the rest of the cheese on top.
Sprinkle with a generous layer of breadcrumbs.
Pour the melted butter over the breadcrumbs as evenly as possible.
Cover and bake for 30 minutes.
Remove the cover and bake for another 15 minutes or until the top is nicely browned.
Let the casserole sit for about 5 to 10 minutes to fully set and rest before serving.
Notes
You can change the cheddar to yellow cheddar and change the pepper jack to Monterey jack if you wish. Adapted from the New York Times.
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