To prepare the parsley butter, combine 1 cup parsley leaves and butter in a food processor, and process until well-blended. Set aside.
To prepare soup, combine water and parsley sprigs in a large saucepan; bring to a boil. Reduce heat; simmer 15 minutes. Strain mixture through a sieve over a bowl, reserving broth; discard solids. Return broth to pan; add potato, carrot, and salt. Bring to a boil. Reduce heat; simmer 15 minutes or until vegetables are tender. Remove 1 1/2 cups vegetables from soup with a slotted spoon. Place remaining soup and vegetables in a food processor; process until smooth, and pour into pan. Return 1 1/2 cups vegetables to soup. Bring to a boil; stir in parsley butter.