In a large saucepan, saute leeks and celery in butter until tender. Stir in the broth, potatoes, carrot, thyme, salt and pepper. Bring to a boil. Reduce heat; cover and simmer for 10-15 minutes or until vegetables are tender. Cool slightly., Transfer to a blender; cover and process until smooth. Return to the pan. Whisk a small amount of soup into buttermilk; return all to the pan, stirring constantly. Add dill; heat through (do not boil). Garnish if desired.