FOUND IT: My mother-in-law spotted it in a Cooks Illustrated publication.
MODIFIED IT: I removed the bread crumb topping, which I've never been a big fan of, and replaced it with some extra cheese on top. Other changes are noted below.
1 lb elbow macaroni
4 tbsp unsalted butter
5 tbsp flour
3 12-ounce cans evaporated milk (I use two and 1 1/2 cups skim milk)
2 tsp hot pepper sauce
1/8 tsp nutmeg
1 tsp dry mustard
8 ounces extra sharp cheddar, shredded
5 ounces American cheese, shredded (You can use mild cheddar, or just skip it and replace it with sharp cheddar)
2 ounces Monterey Jack cheese, shredded
(The recipe calls for you to save 1/2 cup of the pasta water. If you use the skim milk as I did above, you don't need it.)
Boil macaroni in a large pot with 1 tbsp of salt until al dente. Save 1/2 cup pasta water, then drain and rinse with cold water. Set aside.
Melt butter in the now empty pot, cooking until it becomes foamy. Whisk in the flour until it starts to brown a little, then whisk in the milk, hot sauce, nutmeg, mustard, and 2 tsp of salt, stir and simmer until the mixture *starts to* thicken, roughly 4-5 minutes. Off heat, stir in cheeses and reserved cooking water (if you're using it). Stir in pasta and mix until coated.
Pour mixture into 13x9 baking dish, top with some extra shredded cheese (maybe some jack or mild cheddar), and bake at 350 for 20-25 minutes. Let cool 5-10 minutes before serving.