Combine porcini mushrooms and 2 cups hot water in medium bowl. Let stand until porcini mushrooms, soften, about 15 minutes.
In a saucepan melt butter and add carrots, celery, parsnips and shallots. Saute until tender. Add bouquet garni and chicken stock.
Using slotted spoon, transfer porcini mushrooms into saucepan with chicken broth. Add porcini-soaking liquid to saucepan, leaving sediment behind in bowl. Add chestnuts and salt and pepper. Bring to boil, reduce heat cover and simmer for 20 minutes, stirring occasionally. Working in batches, puree soup in blender until smooth, return soup to saucepan. Add Sherry. Bring soup to simmer, correct seasonings adding salt and pepper, to taste. Can be prepared 1 day ahead. Cool slightly and refrigerate. Re-warm over medium heat before continuing.
Ladle soup into bowls. Top with vegetables, and garnish with leeks.