Melt butter and olive oil in a large soup pot over medium heat. When the butter has melted, add onion and cook, stirring occasionally, until onion is completely softened, about 4 to 5 minutes. Stir in the pumpkin and cook another 3 minutes. Season well with salt and pepper. Stir in the chicken broth and bring the soup to a simmer, stirring occasionally. Reduce heat to low and cook 10 minutes. Add brown sugar, crushing it first in your hands to smooth out the lumps. Mix well. Meanwhile, make the Herbed Goat Cheese Mousse: Using an electric or stand mixer, whip the cream until it holds together in soft peaks. With the machine off, crumble in the goat cheese, salt, pepper, and thyme. Turn machine on to medium speed and whip goat cheese and cream together until uniform, about 1 minute, scraping sides of bowl as necessary. Chill until ready to use. The soup and the goat cheese mousse can be made several hours ahead of time. Reheat soup and serve hot, topping each bowl with a tablespoon-sized dollop of Herbed Goat Cheese Mousse. Serves 4 to 6.