Melt butter in the top of a double saucepan. Add flour and blend well. Add milk and stir until sauce begins to thicken; then add grated Parmesan and stir until sauce is smooth and thick. Season to taste with mustard, cayenne pepper and salt.
Cook spaghetti in boiling salted water until tender. Drain well. Separate eggs and stir yolks, one by one, into slightly-cooled cheese mixture. Stir spaghetti into cheese mixture. Beat egg whites until they are stiff, but not dry. Fold gently into spaghetti and cheese mixture and pour into a well-buttered soufflé dish. Cook in a pre-heated