Melt butter in heavy large saucepan over medium heat. Add carrots, onion and dill seeds and sauté until onion is translucent and tender, about 10 minutes. Add 4 cups broth and bring to boil. Reduce heat and simmer until carrots are very tender, about 35 minutes. Transfer soup to blender in batches and puree. Thin with more broth if desired. Season to taste with salt and pepper.
Combine fresh dill and pine nuts in processor and chop finely using on/off turns. With processor running, slowly add oil and process until well blended. Season to taste with salt and pepper. (Soup and pesto can be prepared 1 day ahead. Cover separately and refrigerate.)
Ladle into bowls. Divided dill pesto among bowls. Swirl pesto into soup. Top with a dollop of sour cream.
Adapted from Bon Appétit, August 1993