Chestnut Soup With Grappa Cream

Name: Chestnut Soup With Grappa Cream
Ingredients: 2 tablespoons unsalted butter; 2 ounces thinly sliced pancetta; 2 medium shallots, sliced (1/2 cup); 1/2 cup diced peeled celery root; 1/2 pound white mushrooms, sliced; One 14-ounce jar vacuum-packed roasted chestnuts, coarsely chopped; 4 cups chicken stock or low-sodium broth; 1/4 cup Cognac; 1 fresh bay leaf; 1 sage sprig; 1 thyme sprig, plus 1 tablespoon thyme leaves, for garnish; 3/4 cup half-and-half; Salt and freshly ground white pepper; 1 cup heavy cream; 2 tablespoons grappa