Put the zucchini, broth and basil in a saucepan, bring to a gentle boil, then simmer, uncovered, about 15 minutes. Cool before transferring in batches to a processor or blender; puree until smooth. Taste for seasoning. Refrigerate several hours or several days. Stir well before ladling into bowls. Serve with a dollop of sour cream and basil leaf garnish.
Makes 6 servings