Heat the oven to 400°F/200°C. Score the duck breast and render the fat in a sauté pan. Pour off the fat, and transfer to the oven to finish, about 10 minutes. Remove the duck, and set aside to rest while you make the sauce.
Deglaze the sauté pan with the Cognac and wine and reduce by half. Add the peppercorns and veal stock and reduce by half again. Remove from the heat and whisk in the butter.
Carve the duck breast. Spoon over the sauce and serve.