Preheat the oven to 375 degrees F (190 degrees C). Pour a thin layer of water in a baking dish, or a cookie sheet with sides. Place the squash halves cut side down on the dish. Bake for about 40 minutes, or until a fork can easily pierce the flesh. Cool slightly, then remove the peel. Set aside.
Melt the butter in a large pot over medium heat. Add the onion, leek and garlic, and saute for a few minutes, until tender. Pour the chicken broth into the pot. Add the potatoes, and bring to a boil. Cook for about 20 minutes, or until soft. Add the squash, and mash with the potatoes until chunks are small. Use an immersible hand blender to puree the soup, or transfer to a blender or food processor in batches, and puree until smooth. Return to the pot.
Season the soup with cayenne pepper, allspice, nutmeg, ginger, salt and pepper, then stir in the sherry and half-and-half cream. Heat through, but do not boil. Ladle into bowls, and top with a dollop of sour cream.