Preheat the oven to 350 degrees F (175 degrees C). Spray a 9x13-inch baking dish with cooking spray.

Bring a large pot of water to a boil. Add macaroni and 2 1/2 teaspoons salt and cook, stirring frequently, for 5 to 6 minutes. Drain and set aside.

Shred Cheddar cheese by hand or in a food processor; set aside.

Melt butter in a large saucepan over medium heat. Whisk in flour and cook, whisking constantly, until smooth, 3 to 4 minutes. Whisk in mustard powder, then 3 1/2 cups milk. Whisk occasionally until mixture comes to a simmer, then let simmer for 5 minutes.

Add cream cheese and cook, stirring often, until thick, creamy, and smooth, about 5 minutes. Add remaining 1 1/2 teaspoon salt, pepper, and cayenne.

Stir in 2 cups shredded Cheddar. Turn off the heat and let cheese melt. Stir in cooked macaroni. Add up to 1/4 cup milk if you feel like the mixture is too thick.

Layer 1/2 of the pasta mixture in the bottom of the prepared baking dish. Top with 1 1/2 cups Cheddar. Spread remaining pasta mixture over top and sprinkle with remaining Cheddar.

Bake in the preheated oven until golden and bubbly, 25 to 30 minutes.