Poblano chiles vary in heat from mild to hot; you may need more or less to taste. If you like, garnish each serving with additional cooked corn kernels.
Heat oil in a medium saucepan over medium heat. Add onion, poblano chile, and garlic; saute until onion is soft and translucent and chile and garlic are tender and fragrant, about 5 minutes.
Add coriander, cumin, and turmeric, and cook until they are toasted and fragrant, about 2 minutes. Add corn, and saute until kernels are lightly browned, about 5 minutes. Remove from heat, and let cool slightly.
Transfer 1 1/2 cups corn mixture to the bowl of a food processor fitted with the metal blade, and add buttermilk and salt; puree until mixture is smooth. Transfer to a large bowl or plastic storage container; stir in remaining corn mixture. Cover with plastic wrap, and place in refrigerator until soup is well chilled, at least 2 to 3 hours. Remove from refrigerator; ladle into bowls, and serve.
This soup can be made up to one day ahead and stored, covered, in the refrigerator; stir it again before serving, as it may separate while it sits.