Melt butter in a Dutch oven
Add leeks and celery and sauté 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 egg yolks into the cream cheese and yogurt mixture.
Slowly whisk in 2 cups leek mixture to temper the egg yolks before adding to the soup.
Stir cream cheese mixture and white pepper into leek mixture.
Cook over low heat, stirring occasionally, until thoroughly heated. (Do not boil.)
Sprinkle with chives, if desired.