Austrian Cream Cheese Soup

Melt butter in a dutch oven; add leeks and celery; saut\xe9 until tender. Add flour, cook, stirring constantly, 3 minutes. Stir in broth, 3 cups water, and salt; bring to a boil. Reduce heat; simmer, stirring occasionally, 15 minutes. Microwave cream cheese and yogurt in a 2-quart microwave-safe bowl at High 15 to 20 seconds or until warm; whisk until smooth. Whisk in egg yolks; gradually whisk in 2 cups leek mixture. Stir cream cheese mixture and pepper into leek mixture. Cook over low heat, stirring occasionally, until thoroughly heated. (Do not boil.) Sprinkle with chives, if desired