Roast Duck with Grape Sauce and Crispy Chestnuts

Name: Roast Duck with Grape Sauce and Crispy Chestnuts
Ingredients: 1 medium (5-6 lb.) whole duck; Sea salt and freshly ground black pepper, to taste; 2 cups chicken stock; 2 tbsp cold unsalted butter, divided; 1/2 cup minced shallots; 1 cup madeira wine, divided; 2 tbsp honey; 2 tbsp sherry vinegar; 1 tbsp cornstarch dissolved in 1/4 cup water; 6 bay leaves; 2 ribs of celery, cut in thirds; 1/2 lb seedless green grapes, halved; 1/2 lb seedless red grapes, halved; 1 lb fresh or jarred chestnuts, lightly roasted, shelled, skinned, and coarsely chopped