Preheat oven to 400°. Spray a 3-quart baking dish with cooking spray.
In a large stockpot, cook pasta according to package directions, adding 1 teaspoon salt. Drain, and place in a large bowl.
In a large saucepan, melt butter over medium heat. Whisk in flour until smooth; cook, stirring constantly, for 2 minutes. Gradually whisk in warm cream, warm milk, dry mustard, pepper, and remaining 2 teaspoons salt; cook, stirring frequently, until thickened, about 5 minutes. Reduce heat to low; stir in 2 cups Cheddar and mozzarella until smooth. Pour cheese sauce over pasta, and stir until combined.
Transfer pasta mixture to prepared pan. Top with bread crumbs and remaining 1 cup Cheddar.
Bake until top is golden brown, 10 to 15 minutes. Let cool for 10 minutes before serving.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/classic-mac/