1 tablespoon coconut oil (see Note) or vegetable oil
3 1/2 cups unsweetened coconut milk
1/4 cup plus 2 tablespoons dark brown sugar
1/4 cup Asian fish sauce
1/4 cup tamarind concentrate (see Note)
6 fresh kaffir lime leaves (see Note)
3 tablespoons fresh lime juice
Salt and freshly ground pepper
3/4 pound medium shrimp, shelled and deveined
Cooked rice vermicelli or steamed rice, for serving (optional)
In a blender, combine the chiles, lemongrass, garlic, ginger, shallot and 1/4 cup of water and puree until smooth.
In a medium saucepan, heat the coconut oil. Add the lemongrass puree and cook over moderately high heat, stirring, until fragrant, about 2 minutes. Whisk in the coconut milk, brown sugar, fish sauce, tamarind, lime leaves and 1 1/2 cups of water and bring to a gentle boil. Simmer over low heat, stirring occasionally, until the soup is flavorful and slightly reduced, about 15 minutes.
Stir the lime juice into the soup and season with salt and pepper. Add the shrimp and simmer until slightly curled and just cooked through, about 1 minute. Ladle the soup into bowls, add the rice noodles and serve.
Coconut oil, tamarind concentrate and kaffir lime leaves are available at specialty food stores like New York City's Kalustyan's (kalustyans.com) and at many southeast Asian markets.