Preheat the oven to 350°F and spray a 9x13-inch baking dish with nonstick cooking spray.
In a medium saucepan over medium heat, melt the butter.
Add the flour and whisk constantly for 1-2 minutes or until the mixture thickens and is fully incorporated. (You are creating a roux.)
Remove from heat and gradually pour in the warm milk, whisking constantly.
Return the pan to the heat, whisk another 3-5 minutes or until thickened.
Add the salt, garlic powder, ground mustard, and cayenne pepper and stir to combine. Remove from heat.
Add the cream cheese, 3 cups of sharp cheddar, and mozzarella. Stir to melt.
In a large bowl, add cooked noodles, cheese sauce, chicken, diced tomatoes, salt, and pepper. Toss to coat.
Pour into the prepared baking dish and top with the remaining 1 cup of cheddar cheese.
Bake uncovered for 25-30 minutes, or until cheese is hot and bubbly.
Garnish with parsley (optional) and serve hot.