Fill a large pot with water and bring to a boil. Add a small handful of salt and stir in the pasta. Boil, stirring occasionally, until pasta is fully cooked, 7 to 8 minutes.. Drain pasta, reserving 2 tablespoons pasta water. Set aside.
Combine evaporated milk, pasta water, egg, dry mustard, salt, and pepper and whisk together.
Place a large saucepan over low heat and melt butter.
Stir in milk mixture and bring to a gently simmer. Whisk together for about 2 minutes.
Whisk in small handfuls of the cheese mixture at a time, making sure not to add more cheese until the previous addition had been fully incorporated to the sauce.
Continue to whisk and add cheese over low heat until all the cheese has been added and a creamy and smooth sauce forms (make sure not to rush this process or your cheese sauce will be gritty).
Once the sauce has been made, stir in cooked pasta. Add more salt and pepper as needed and serve.