Lightly butter a 2 quart baking dish with 1 tablespoon of softened butter. Set aside.
Fill a large pot with water and bring to a boil. Add a handful of salt and pasta and boil, stirring occasionally, until pasta is fully cooked. Drain pasta, reserving 3 tablespoons pasta water. Set aside.
In a large saucepan combine evaporated milk, pasta water, egg, dry mustard, salt, and pepper and whisk together. In a large mixing bowl combine cheese, starch and lemon juice and toss together.
Place saucepan over medium-low heat and whisk together for about 2 minutes. Whisk in small amounts 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 the low heat until all the cheese has been added and a smooth and creamy sauce forms (make sure not to rush this process or your cheese sauce will not be smooth).
Once the sauce has been made, stir in the pasta. Adjust seasonings.
Pour mixture into a lightly buttered baking dish.
In a small bowl combine crushed crackers and butter. Sprinkle mixture over mac and cheese and broil on high for 1 to 2 minutes or until crackers have toasted and mac and cheese is bubbling.
Top mac and cheese with chives and serve.