Cook macaroni according to package directions; drain and set aside. Heat oven to 350 degrees. In a large bowl, mix dry milk, flour and butter. Gradually add boiling water, beating constantly. Add 1+ cups cheese and continue beating until smooth and creamy. Stir in macaroni, 1 cup of the remaining cheese and salt. Transfer to lightly greased 2-quart baking dish. Cover with foil.
Bake 25 minutes. Remove foil. Sprinkle with remaining + cup cheese. Continue baking 1 minute, or until cheese melts.