Trim, wash and slice zucchini. Put in saucepan with broth and marjoram. Cook until tender (about 20 minutes). Puree through a food mill or in a blender. Melt the butter or margarine and add flour to make a creamery mixture. Cook, stirring 2 or 3 minutes, and remove from the heat. Cool slightly. Heat milk until it comes to a boil. Add the milk to the cooled butter mixture. Return to heat and cook, stirring for 15 minutes or until flour taste is gone. Salt and pepper. Add to pureed zucchini and chill thoroughly. Stir cream into chilled soup before serving. Garnish with parsley.