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.
Turn off the heat, and remove the skillet from the stove**. 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.
*Be sure to buy a good quality block of sharp cheddar cheese, then grate it yourself. Those bags of pre-shredded cheese have additives in them that are specifically there to prevent clumping, so if you don’t shred your own cheese, the sauce won’t be smooth. My favorite brand cheddar is Cracker Barrel.**A few people have reported a gritty/grainy texture, which is usually a result of cheese being overheated. I have added this step of removing the skillet from the heat, especially if you have an electric stove, so the cheese does not overcook. The residual heat of the béchamel should melt the cheese sufficiently. You definitely don't want the cheese to bubble.To reheat: ONLY reheat on the stovetop in a pan (no microwave here). Heat the cheese in a skillet over medium low heat until melted and warm. Add 2 tbsp of water and whisk until the cheese looks smooth and creamy again (adding moisture should restore that creamy smooth texture).