Preheat oven to 450 degrees.
Wrap each wedge of squash tightly in foil.
Place foil packages on oven floor. Remove after 30 minutes.
Melt butter in a 2-quart heavy-bottomed saucepan, and gently cook onion and garlic over medium-low heat until soft, about 10 minutes.
Add chicken broth to onion mixture and simmer 10 minutes.
Scoop out flesh from squash (discard seeds and skin) and add to soup.
Add salt, pepper, and sugar. Simmer 10 minutes.
Remove from heat and let cool.
Puree soup in food processor or blender, and stir in cream, cinnamon, and mace. Serve hot.