In a sauce pan, heat butter and cook garlic, onion and celery until tender. Add sweet potatoes. Saute for a couple of minutes.
Add water and vegetable bouillon and bring to boil. Cook over medium heat until sweet potatoes are tender.
Transfer to the blender. Puree all vegetables. Then transfer to the sauce pan again and heat over low heat, season with salt and pepper and simmer for about 5 minutes. Add sour cream and stir well. Turn off the heat. When ready to serve, sprinkle minced parsley.