Bring broth to boil in large saucepan. Add sweet potatoes and curry powder; cover and simmer until potatoes are very tender, about 20 minutes. Using slotted spoon, transfer sweet potatoes to processor. Puree until smooth. Add 1 cup broth; process until well blended. Return mixture to saucepan with broth. Add milk; bring to simmer. Season with salt and pepper. (Can be made 1 day ahead. Chill. Rewarm over medium heat.) Ladle soup into bowls. Top with dollop of yogurt. Sprinkle with chives.
