Bring the water, 1 cup of the milk, macaroni, and dash of salt to a simmer in a large nonstick skillet over medium-high heat. Cook at a simmer, stirring often, until the macaroni is tender and the liquid has thickened, about 9 to 12 minutes.
In a measuring cup, whisk the remaining milk, cornstarch, and mustard together. Stir into the skillet and simmer until slightly thickened, about 1-2 minutes.
Take the pan off the heat and stir in the shredded cheese, one handful at a time. Stir in the butter until combined and season with salt and pepper to taste.
Slightly adapted from a recipe found here.
Notes
I use a 3 quart saute pan when I make this recipe, so make sure you use a large enough skillet. Also, if you have leftovers, you can heat them up in a saucepan on the stove and add a little extra milk as needed to make it creamy again.Slightly adapted from a recipe found here.