In a large saucepan, melt 1 tablespoon of the butter. Add the shallots and celery root and cook until the celery root is just tender, about 10 minutes.

Transfer the vegetables to a bowl, leaving the fat in the saucepan.

Add the remaining 1 tablespoon of butter to the saucepan along with the mushrooms and all but 2 tablespoons of the chestnuts. Cook over moderately high heat, stirring occasionally, until the mushrooms have released their liquid and are slightly softened, 6 minutes.

Add a little of the chicken stock and the celery root mixture and cook for 1 minute.

Add the Cognac and cook until evaporated, 2 minutes.

Add the bay leaf, sage and thyme sprigs to the saucepan along with the remaining stock (minus 1 cup or so) . Bring to a boil and simmer for 30 minutes, stirring occasionally.

Discard the bay leaf, sage and thyme.

Working in batches, puree the soup in a blender until creamy and smooth. Use the remaining stock to "rinse" the blender and return with the rest of the soup to the saucepan. Stir in the milk and heavy cream; season with salt and white pepper and keep warm.

To serve, ladle the soup into small bowls.

Dollop the grappa cream on top, garnish with the thyme leaves and the reserved 2 tablespoons of chestnuts and serve.