Mary told me the recipe for this off-the-cuff classic, and it was fantastic.
1 can evaporated milk
2 egg yolks
Worcestershire sauce (optional)
Tabasco sauce (optional)
While cooking the macaroni (enough for however many people you're serving) combine the evaporated milk and the egg yolks, add the two sauces to taste if you like, I forgot to and it was still delish. Just before the macaroni is done, drain it and add the milk/egg mixture. Cook until it is somewhat absorbed, and add cheese. I like a lot of cheese, and just used the remainder of the block in the fridge. Salt to taste.
These directions are pretty general and do involve some estimation, that's how I generally do things. It's pretty foolproof, and is creamy even when reheated.