Melt the butter in a medium size skillet over medium heat. Once the butter is melted add the flour. Whisk until it combines into a smooth blond paste and starts to bubble around the edges.
Add the milk to the skillet and continue whisking. Once the milk is incorporated, raise the heat to high and bring to a boil. Once the milk has reached a boil, quickly lower the heat and reduce to a simmer. Allow the sauce to thicken slightly.
Add the cayenne pepper and salt and whisk to combine.
Add the cheese and gently stir, allowing the cheese to melt. When the cheese is completely melted and the mixture smooth in consistency, the nacho cheese sauce is ready to serve. Enjoy immediately, as the sauce will get firm as it sits.