Bring a large pot of salted water to a rolling boil, add the shell pasta, and cook until al dente according to package directions. Before draining, scoop out 1/2 cup (120ml) of starchy pasta water and set aside, then drain the pasta without rinsing.
Shred 2 cups (225g) of sharp white cheddar from the block using a box grater, and tear or thinly slice the white American cheese into smaller pieces. Set both cheeses aside at room temperature so they melt smoothly into the sauce.
Combine the whole milk and heavy cream in a small saucepan or microwave-safe jug and heat gently until warm to the touch but not boiling, about 60-90 seconds in the microwave. Warmed dairy prevents lumps when added to the hot roux.
Melt the butter in a heavy-bottomed saucepan over medium-low heat, then add the flour all at once and whisk constantly for 60-90 seconds until the mixture smells slightly nutty and looks like a smooth pale golden paste.
Slowly pour the warmed milk and cream into the roux in a thin, steady stream while whisking constantly, then increase heat to medium and cook for 4-6 minutes, whisking often, until the sauce is thick enough to coat the back of a spoon.
Remove the saucepan from the heat, then stir in the American cheese until melted, followed by the white cheddar in two batches, stirring between each addition. Once smooth, stir in the Dijon mustard, salt, pepper, and hot sauce, then taste and adjust seasoning.
Return the saucepan to very low heat and fold the drained pasta into the cheese sauce using a spatula until every piece is evenly coated, adding reserved pasta water a tablespoon at a time if the sauce is too thick. Cook for 1 minute until heated through.
Ladle the mac and cheese into bowls and serve right away for the creamiest texture, topped with extra shredded white cheddar, a drizzle of hot sauce, fresh parsley, or crispy breadcrumbs if desired.