In small cup, dissolve dry mustard in water. Set aside.
In medium bowl, whisk together eggs, 1 cup of evaporated milk, dry mustard mixture, salt and pepper. Set aside.
Bring 2 quarts of water to boil in large Dutch oven or pot. Add pasta and 1½ teaspoons salt and cook, stirring often, until al dente. Drain pasta and return to pot over low heat. Add butter and stir until completely melted.
Add egg mixture and three-quarters of cheese mixture to pasta and toss until thoroughly combined and cheese starts to melt. Gradually add remaining evaporated milk and remaining cheese mixture, stirring constantly, until mixture is hot and creamy, about 5 minutes. Serve immediately.
Pesto Mac & Cheese: Mix in 1/2 cup prepared pesto, 1/3 cup sun-dried tomatoes (slivered), 3/4 cup corn kernels (cooked).
Bacon, Artichoke & Spinach Mac & Cheese: Mix in 4 cups baby spinach, 1/4 cup bacon (cooked and crumbled), 1/2 cup artichoke hearts (roughly chopped).
Buffalo Chicken Mac & Cheese: Mix in 1 cup boneless skinless chicken breasts (shredded), 1/2 cup blue cheese (crumbled), hot sauce (to taste).