Preheat oven to 375°F. Oil baking sheet. Place squash, cut side down, on baking sheet. Bake until squash is very soft, about 50 minutes. Using paring knife, remove peel from squash; discard peel. Set aside
Heat butter in heavy large pot over medium-low heat. Add onion. Cover pot and cook until onion is tender and slightly brown, about 15 minutes. Add spices, squash and 4 cups chicken broth. Bring to boil. Reduce heat to medium-low. Cover and simmer 10 minutes.
Working in batches, purée soup in blender. (Can be prepared 1 day ahead. Cool slightly. Cover and refrigerate.) Return soup to pot. Season soup with salt and pepper. Bring to simmer, thinning soup with more broth if necessary. Ladle into bowls. Add a drizzle of cream and top with pumpkin seeds. Serve.