Bring a large pot of salted water to a boil. Add the pasta and cook it until al dente, about 10 minutes. Drain the pasta then set it aside.
Add the milk to a medium saucepan set over low heat. Once the milk is hot to the touch, turn off the heat and set it aside.
Add the butter to a large heavy-bottomed stockpot set over low heat. Once the butter has melted, whisk in the flour. Cook the mixture, whisking constantly, until it is pale golden in color and smells slightly toasted, about 3 minutes.
Slowly whisk in the warm milk, whisking constantly to avoid any lumps. (The mixture will become very thick then thin out slowly.)
Increase the heat to medium and continue whisking the mixture constantly until it has thickened enough to coat the back of a spoon. Whisk in the salt and pepper.
Stir in the white cheddar cheese and Parmesan cheese and continue cooking, stirring frequently, until the cheese sauce is creamy and the cheese has melted.
Stir in the drained pasta then taste and season the mac and cheese with salt and pepper. Serve.
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