Heat the oven to 400 degrees F/200 degrees C. Score the duck breast and render the fat in a saute 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 saute pan with the wine and cognac and reduce by half. Add the veal stock and peppercorns and reduce by half again. Remove from the heat and whisk in the butter.
Carve the duck breast. Spoon over the sauce and serve.