Preheat oven to 350º
Cut the squash in half lengthwise and scoop out the seeds.
Place the squash halves skin side down in a large roasting pan. Place a pat of butter (about ½ tablespoon) and 1 teaspoon brown sugar in the cavity of each.
Arrange the carrots and onion slices around the squash. Pour 2 cups of chicken stock in the pan. Cover tightly with aluminum foil and bake for 2 hours.
Allow vegetables to cool slightly, and then scoop out the squash flesh and put in a soup pot. Add the carrots, onions and cooking liquid.
Add the remaining 8 cups chicken stock, mace, ginger, cayenne and salt. Bring to a boil and then reduce heat and simmer for 10 minutes.
Purée the soup in batches and return it to the soup pot. (Use a blender, food processor, immersion blender or food mill.) Adjust seasonings and heat through. If you use a food mill, don't forget to scrape the bottom of the mill, lots of good stuff there.
Serve the purée garnished with snipped chives and a dollop of crème fraiche, sour cream or drizzle of heavy cream.