In a large non-stick pot, bring water, 1 cup of evaporated milk, macaroni, and salt to a boil over high heat, stirring often. Turn heat down to medium - medium-high and boil for 9 minutes, stirring occasionally.
Meanwhile, whisk cornstarch into remaining evaporated milk with dry mustard (other spices can be added here to your preference, like garlic powder, onion powder, cayenne, or paprika).
Add milk and cornstarch mixture to noodles. Reduce heat to medium-low. Stir in cheese and butter until smooth and creamy. If mixture is thick or stringy, add water by the 1/4 cup to smooth out. *When water is added be sure to add more salt.
Taste for seasoning and add more salt and pepper as needed. Serve immediately.