Melt the butter in heavy sauce pan over medium heat. Add carrots, onion and dill seed and saute until onion is trancslucent and tender, about 10 minutes. Add 4 cups chicken broth and bring to a boil. Reduce heat and simmer until carrots are very tender, about 35 minutes. Puree with a stick blender. Thin with more chicken broth if desired. Season to taste with salt and pepper.