This is one of my favorite soups. From Robuchon:
Melt 1 tablespoon of butter in a pot over low heat. Add 400g shelled chestnuts and saute for 3 minutes. Add 1 liter of water, along with a few pinches of fleur de sel and white pepper. Add the seeds of 2 cardamom pods, bring to a boil and then simmer until the chestnuts are completely tender. Blend for 2 minutes (you may need to adjust the consistency by adding a little water), then add 3 tablespoons of cold butter, 1/3 cup of cream, and one egg yolk. Strain, and check for seasoning. Garnish with cubes of cooked celery root, bacon, foie gras (optional), and celery leaves.