Trim off the root ends of the leeks and top of the green parts. Split leeks in quarters and wash thoroughly under cold running water. Drain, pat dry and chop. (There should be about 4 cups.) Heat the butter/margarine in a heavy kettle and add the leeks. Cook, stirring for about 5 minutes. Drain the potatoes and add them to the pot. Stir. Add the chicken broth and black pepper; cook uncovered for about 30 minutes. Ladle the mixture into the container of a food processor or electric blender. Process into a fine puree, transfer to a bowl, cover, and chill. When ready to serve, stir in the milk and yogurt and blend well. If the mixture thickens as it stands, thin with unsalted chicken broth. Serve sprinkled with the chives.
Makes about 10 servings