1. Grease a 2-quart soufflé dish with 1 tablespoon margarine; coat bottom and sides with breadcrumbs. Set aside.
2. Bring 2 quarts water to a boil in a 3-quart saucepan; stir in shell macaroni from package, and set cheese sauce aside for later use. Cook 15 minutes; drain. (Do not rinse.) Return to pan. Stir in 1/4 cup margarine, Parmesan cheese, and 1/2 cup milk, stirring until cheese melts. Let cool slightly.
3. Process macaroni mixture in a food processor, adding remaining 1 cup milk in a slow, steady stream, processing until almost smooth. Add cheese sauce, egg yolks, and next 4 ingredients; process 30 seconds, stopping to scrape down sides.
4. Beat egg whites at high speed with an electric mixer until stiff peaks form. Gradually stir about one-fourth of cheese mixture at a time into egg whites. Spoon into prepared dish; place dish on a baking sheet. Bake at 350° for 1 hour. Serve immediately.