How to Make It
Melt the butter in a large saucepan. Stir in the onion and garlic, cover and cook over low heat, stirring occasionally, until softened, about 15 minutes. Add the curry powder and stir over moderate heat for 1 minute. Stir in the squash, add the stock and simmer over moderate heat until the squash is soft, about 25 minutes.
Puree the soup in batches in a blender or food processor. Pour the soup into a clean saucepan and stir in the buttermilk and half-and-half. Season with salt and pepper and simmer just until heated through. Ladle the soup into shallow bowls, sprinkle with the cilantro and serve.