Add the flour to a small skillet. Pour in the milk, and whisk quickly until the two are completely combined without lumps. Heat over medium heat.
Add the cheese, butter, and salt to the mixture. Whisk constantly until the cheese begins to melt.
As soon as the cheese begins to melt, reduce the heat to a bit above low and let the cheese sauce simmer for 30 minutes. Whisk every few minutes to encourage melting, while the sauce comes together and thickens.
When the sauce begins to thicken, prepare the pasta according to the boxes directions. If finished before the sauce, strain, and run under cool water to stop cooking and prevent sticking. Transfer the pasta to a mixing bowl.
Once the cheese sauce is done simmering and thickened finally, pour it over the pasta. Stir until the pasta's evenly & completely coated. Transfer this to a casserole dish that's been lightly sprayed with non stick cooking spray.
Bake the mac and cheese at 350 degrees for 25 minutes.
Remove from the oven, let the dish rest 60 seconds, and serve immediately.