Rich in taste and texture, the woodsy, earthy notes of this elegant soup make it a memorable first course for a winter dinner. If you have the budget for splurging on wild mushrooms, by all means substitute fresh chanterelles, shiitakes, or porcinis (or an assortment) for the button mushrooms in this recipe. The dried porcini are central to the deep flavor of the bisque, so don't eliminate these. Chop the mushrooms in a food processor (in batches) to save time; since the bisque is a purée, the uniformity of your slicing and chopping isn't an issue for presentation.There are many garnishes other than those suggested here that also partner beautifully with this soup, such as a thin slice of chevre (goat cheese), a dollop of crème fraîche or mascarpone with a sprinkling of chives, or a drizzle of cream. Any leftover soup can be carefully re-heated the next day.
Place the dried porcini mushrooms in a bowl and add 2 cups of hot stock. Let the mushrooms sit at room temperature to soften for 20 minutes.
Heat the butter in a large pot or Dutch oven over medium heat. Add the leeks and shallots and cook, stirring frequently, until they soften, about 5 minutes. Add the wine and brandy and cook 2 minutes.
Drain the porcini, reserving the soaking liquid. Chop the porcini and add them to the pot along with the chopped button mushrooms. Add the remaining 4 cups of stock, along with the porcini soaking liquid (leaving behind any sediment).
Bring the mixture to the start of a boil, lower the heat, and simmer the soup, uncovered, until the mushrooms are very tender, about 45 minutes. Let cool slightly.
Working in batches, purée the soup in a blender or food processor until very smooth. Return the soup to the pot and add the cream. Reheat over low heat until the bisque is hot. Season to taste with pepper, truffle salt, if using, or regular salt, to taste.
Ladle into bowls and garnish each serving with a drizzle of truffle oil (if you're not using truffle salt) and a sprinkling of tarragon. Serve piping hot.