Combine porcini mushrooms and 2 cups hot water in medium bowl. Let stand until porcini mushrooms, soften, about 15 minutes. To accellerate you can boiled for 1 min and then tunr it off.

In a saucepan melt butter and add carrots, celery, parsnips and shallots. Add bouquet garni. Saute until tender. (Usually 3 min at medium heat). 

Using slotted spoon, transfer porcini mushrooms into saucepan.
Add porcini-soaking liquid to saucepan, leaving sediment behind in bowl.

Prepare the Chestnuts.
In this recipe I used the bottled ones because I think that you can save 1 hour of good work if you find the "Ready to Use" chestnuts like this ones.
The bottled chestnuts are already cleaned and boiled. The only thing you have to do is puree or finely chopping using a food processor.

I like it finely chopped; so the little peices of chestnut will remain visable in the soup.

Add chicken broth.
Add chestnuts and salt and pepper.
Bring to boil, reduce heat cover and simmer for 20 minutes, stirring occasionally.

After 20 minutes stop the cooking.

Using an handblender directly in the saucepan puree the soup until smooth.

Keep blending till reach the level of a cream.

Add Sherry (optional).
Bring soup to simmer, correct seasonings adding salt and pepper, to taste.
The soup 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.
The creamy taste is reached directly using the handblender and without add any cream as other recipe suggest. In this way is lighter than other recipes I tried already.