Cold Avocado Corn Soup With Cilantro Oil

Name: Cold Avocado Corn Soup With Cilantro Oil
Ingredients: 1 fresh or frozen ear of corn, shucked; 4 cups plus 2 tablespoons water; 1 garlic clove, smashed; 1 1/2 teaspoons salt; 1 1/4 cups chopped white onion; 1 fresh serrano chile, stemmed and coarsely chopped (including seeds); 2 firm-ripe California avocados (1 to 1 1/4 lb total); 3 tablespoons fresh lime juice; 1/4 cup crema or sour cream; 1 cup coarsely chopped fresh cilantro; 1/4 cup olive oil; 1/2 teaspoon salt; a 3/4-inch melon-ball cutter