Cook the pasta. Bring about 4 quarts of salted water to a boil over high heat in a large pot. Add the pasta and cook according to package directions. Drain and set aside.

Place 1 cup of the milk in a medium saucepan over medium heat. Meanwhile, place the remaining 1/2 cup of milk and flour in a small bowl and whisk together until there are no lumps. When you just start to see whisps of steam rising from the warming milk, whisk in the milk-and-flour mixture. Continue whisking gently until the milk thickens slightly to the consistency of heavy cream, 3 to 4 minutes.

Turn the heat to low and begin mixing handfuls of cheese into the milk. Stir in the salt. Stir until all the cheese has melted and the sauce is creamy. Taste and adjust the seasonings as desired. Remove the sauce from the heat.

Place the pasta and 1/2 of the cheese sauce in a large serving bowl. Stir to coat the pasta evenly. Add the remaining sauce and any extra add-ins and stir to combine. If you'd like a looser sauce, add up to another 1/4 cup milk if desired.

Serve the mac and cheese immediately while still warm.