Preheat oven to 375 degrees. Grease 2-quart casserole dish with cooking spray
Bring 4-quarts water to a rapid boil. Add 1 tsp salt to the water. Add the dry pasta to water, and return to boil. Cook uncovered, stirring occasionally for 5-6 minutes. Drain well, set aside.
Shred 12 oz of sharp cheddar cheese, and set aside.
Combine 1 ½ Tbsp cornstarch, 1 tsp salt, ½ tsp dry mustard, and ¼ black pepper in a medium saucepan.
Stir in 1 can evaporated milk, 1 cup water, and 2 Tbsp butter. Cook over medium heat, stirring constantly, until it comes to a boil. Boil for 1-2 minutes, continue to stir.
Remove from heat, and stir in ⅔ of the shredded cheese until melted. Add cooked macaroni; mix well.
Pour ½ of macaroni into prepared casserole dish; sprinkle with ½ of the remaining shredded cheddar cheese. Add remaining ½ of macaroni, and top with remaining shredded cheese.
Bake 20-25 minutes at 375 or until cheese is melted and light brown on top.