• 8 Tbsp. (1 stick) unsalted butter
• 1 (2-ounce) chunk pancetta
• 2/3 cup chopped shallots (about 3 large shallots)
• 2/3 cup chopped celery root
• 1/2 pound button mushrooms , trimmed and sliced
• 1 1/2 pounds (4 cups) dry-packed chestnuts , coarsely chopped
• 1/2 cup Cognac
• 1 bay leaf , preferably fresh
• 1 sprig fresh sage
• 1 sprig fresh thyme
• 7 cups chicken stock or canned broth
• 3/4 cup half-and-half
• Kosher salt , to taste
• Freshly ground white pepper , to taste

In a large skillet, melt 4 tablespoons butter over medium heat. Add pancetta, shallots, and celery root; cook 5 minutes, or until shallots are softened. Scrape into a large saucepan and set aside.

In same skillet, melt remaining 4 tablespoons butter over medium-high heat. Add mushrooms and chestnuts; cook 5 minutes, or until mushrooms are tender. Remove from heat. Add Cognac and carefully ignite, holding a lighted match just above the surface. When flame dies out, transfer mushrooms and chestnuts to saucepan with pancetta-shallot mixture.

Tie together bay leaf, sage, and thyme; add to mushroom mixture, along with chicken stock. Bring to a boil, reduce heat, and gently simmer 30 minutes, stirring occasionally.

Remove from heat; let cool 10 minutes. Remove pancetta and herb bundle from soup; discard. Working in batches, transfer soup to blender; process until smooth.

Return soup to saucepan over low heat. Stir in half-and-half and cook just until heated through (do not boil). Season to taste with salt and pepper before serving.