Score the skin of the duck breasts with criss-cross incisions. Rub both sides with salt and pepper; set aside. Heat a large frying pan over medium heat until hot. Put in the duck breasts, skin side down and cook until the skin is browned and crisp, 2 to 3 mn. Add the peppercorns. Turn the breasts and continue cooking until medium rare, about 6 mn longer. Pour off and discard fat from pan. Add the brandy, light with a match. When the flames die, remove the breasts; cover with foil and keep warm. Deglaze the pan with the stock. Cook over medium heat until reduced by half. Remove from heat and whisk in the softened butter, little by little, so that the sauce thickens and emulsifies. Slice the breasts diagonally and spoon the sauce over. If you prefer your duck less rare, increase cooking time by 5 mn.