One day in advance: Marinate the grapes in 4 tsp of Armagnac. Place on one side. Dry off the duck breast fillets. Use a sharp knife to cut the skin in a criss-cross pattern, without cutting into the flesh. Drizzle the rest of the Armagnac over the duck breasts. Place aside in a cool place.
On the day of preparation: Chop up the shallots. Preheat the oven, and the tray to 80 °C. Season the duck breast fillets with salt and pepper.Without adding any fat, sear them first on the skin side for 3–4 minutes, and then on the flesh side for 1–2 minutes. Lay them flesh side down on the roasting tray, and cook gently for 50 minutes.
Sauté the shallots in the meat juices in the searing pan until golden brown. Dust the corn flour over them and continue to simmer briefly. Add the red wine. Top up with stock. Simmer for ten minutes over a low heat.
Sieve into another pan. Add the grapes in Armagnac and the green pepper corns. Break up the chocolate and add to the sauce. Dissolve into the sauce over a medium heat. Add salt and pepper to taste. Place on one side.
Remove the duck from the oven. Slice and arrange on warmed plates. Pour the sauce around.