Bring 3 1/2 cups water, 1 cup of the evaporated milk, the macaroni and 1/2 teaspoon of salt to a simmer in a 12-inch non-stick skillet, over high heat. Continue to cook at a vigorous simmer, stirring often, until the macaroni is tender and liquid has thickened. About 12 minutes.
Whisk together the remaining evaporated milk, cornstarch and dry mustard in a small bowl. Stir mixture into the skillet. Continue to simmer another minute or so, until it starts to thicken.
Remove skillet from heat and gradually stir in the 3 cups of cheddar cheese, one handful at a time. Stir until cheese is melted. Stir in the pieces of butter and season with black pepper. Serve immediately.