Heat the olive oil in a large soup pot over medium heat. Add the shallots, garlic, and leek. Cook for 5 minutes, stirring regularly, until softened. Deglaze with brandy, if using, or 3 tablespoons warm water (pour in the liquor or water and scrape the bottom of the pot with a wooden spoon).
If you are using vacuum-packed or jarred chestnuts, pat them dry with paper towels. Add the chestnuts to the pot and cook for 4 minutes, stirring gently from time to time, until the chestnuts are lightly golden.
Reserve two mushrooms for garnish. Slice the remaining mushrooms and add them to the pot. Add the chilli powder if desired. Pour in the stock and season with salt. Bring to a simmer, cover, lower the heat to medium-low, and cook for 20-30 minutes, until the vegetables are soft.
Let the soup cool for a few minutes. Puree the soup using an immersion blender, until the soup is smooth with just a few remaining chunks. Stir in the balsamic vinegar, taste, and adjust the seasoning. (The soup can be made a day ahead up to this point). Reheat and ladle into bowls. Cut the reserved mushrooms in slices, and chop the parsley leaves roughly. Plop a mushroom slice on the surface of each bowl, sprinkle with parsley and a touch of freshly ground pepper, and serve.