Preheat oven to 350. Spray a 9x13 pan with non stick cooking spray.
Bring a big pot of salted water to a boil and cook your pasta in it for about 8 minutes, (check your pasta package directions - I usually do it about 3-4 minutes less than it says to on the package. You don't want it all the way cooked because it is going to continue to cook in the oven.)
While the pasta is cooking, combine your half and half and milk in a microwave safe bowl or jug and cook in the microwave on high for about 4 minutes.
In a large saucepan, (at least 3qt), over medium high heat, melt the butter then stir in the cornstarch. (It will look like a thick yellow paste.) Let it cook for a minute, then whisk in the heated half and half and milk mixture, mustard, salt and pepper. Keep whisking until the sauce starts to bubble and thicken slightly.
Add half of the shredded extra sharp (12oz) and half of the Kraft Creamy Melt Mozzarella (1/2 cup), and stir with a wooden spoon until it's all melted and smooth. Turn off the heat and set aside.
Drain you pasta, return it to the big pot, then pour the cheese sauce all over it and toss it all together.
Pour HALF the cheesy pasta mixture into the prepared pan. Sprinkle half of the remaining extra sharp cheese (6oz) over that half of the pasta mixture, then pour the remaining pasta mixture over the top. Sprinkle with the last 6oz of the extra sharp and the last 1/2 cup of the Kraft Creamy Melt Mozzarella.
Bake at 350 on the middle rack for 15 minutes, then leave the mac and cheese in the oven, but raise the temperature of the oven to 425 and let it bake another 7-10 minutes or until the cheese is nice and bubbly and starting to turn golden brown on top.
Remove from the oven, and let sit for 5-10 minutes before serving.