Melt butter in a 4-quart saucepan over medium-high heat. Add onions and garlic and cook, stirring often, until softened, about 4 minutes. Add spices and stir for a minute more.
Add pumpkin and 5 cups of chicken broth. Bring to a boil and reduce heat, simmer for 10 to 15 minutes.
Transfer soup, in batches, to a blender or food processor. Cover tightly and blend until smooth. Return soup to saucepan, and add brown sugar. Slowly add milk and cream, stirring to incorporate. Adjust seasonings to taste. Re-heat gently.
Serve in individual bowls, and sprinkle with roasted pumpkin seeds.