Place the diced squash on a lined baking tray and sprinkle it with brown sugar, the honey and 2 tablespoons butter.Roast until caramelized and soft to the touch, about 20 minutes.Roast the unpeeled banana in the oven at the same time.Melt the other 2 tablespoons butter in a large saucepan on medium heat and sweat the chopped onion, celery and carrot for a few minutes until tender.
Add the garlic, curry powder, coriander, nutmeg and cinnamon and cook slowly for a few more minutes.When the squash is done, remove it from the tray and add to the soup.Take the banana from its skin and also add it to the soup, along with the chicken broth.You can scrape any sugar/honey mixture left on the tray lining into the soup as well.Simmer for a further 10 minutes, then add the coconut milk and blend until smooth.Thin out the soup, if desired, with a bit more broth.Blend again until smooth and pass the soup through a chinois or household strainer.
Add lime juice and salt and pepper to taste.Ladle into bowls and serve with cilantro leaves as a garnish, a drizzle of truffle oil or a few toasted pumpkin seeds.The soup goes well with a chilled Gewurztraminer wine.