Mix ¼ cup of milk and cornstarch (or all purpose flour) without any lumps, keep this flour milk mixture aside.
Add the pasta, water, one cup of whole milk and butter to a saucepan, stir well and bring the mixture to a boil (stir often).
As the mixture starts boiling, reduce the heat to medium, and add the spice powders - garlic powder, onion powder and mustard powder and cook for 3 to 4 minutes or until the pasta is cooked. Keep stirring continuously to avoid the pasta from sticking to the bottom of the pan.
Add the flour milk mixture also to the pan and cook for another 1 to 2 minutes.
Switch off the stove and keep the pan away from heat. Stir in the heavy cream and cheese (shredded cheddar or Colby if you are using and also the American cheese if using) and stir well everything so the cream and cheese will blend nicely with the hot pasta in sauce.
The mac and cheese will have a tendency to thicken quickly, so add the chilled milk, one cup at a time, and stir well so as to maintain the right consistency. If the pasta thickens again in few minutes (which is normal), add more chilled milk, little by little as needed, and stir well. In case you need to save the pasta for later use, you may add a little more warm milk (if the pasta is already at room temperature or cold)and stir well to reduce the thickness of the dish, if needed.