Preheat oven to 325°F. Wash squash, halve, and remove seeds. Place squash halves, cut sides down, in a 3-quart rectangular baking dish. Bake for 50 to 60 minutes or until tender. Let squash cool about 10 minutes or until cool enough to handle. Using a spoon, remove squash pulp from shells. Discard shells.
In a large saucepan, heat hot oil over medium heat. Add onion; cook until tender, stirring frequently. Add squash pulp, broth, and the water. Cook over medium-high heat until mixture just reaches boiling, stirring frequently. Stir in brown sugar, salt, 1/4 teaspoon cinnamon,garlic powder, and pepper. Remove from heat; cool slightly. Place half of the squash mixture in a blender or food processor. Cover and blend or process until smooth. Pour pureed squash into a medium bowl. Repeat with remaining squash mixture. Return all squash mixture to saucepan. Stir in half-and-half; heat through. If desired, garnish individual servings with pumpkin seeds and sprinkle with additional cinnamon.