1) In a bowl, place together, butter, Parmesan, parsley, garlic, basil, and marjoram.
2) Using a wooden spoon stir the cheese mixture until well blended.
3) Slowly stream in the oil, beating continuously until all the oil is incorporated.
4) Mix in the walnuts and stir well.
5) Place the warm spaghetti on the heated serving plate.
6) Stream the sauce over the spaghetti and toss lightly until well coated.
7) Serve hot garnished with sprig of parsley if desired.