How to Make It
Score the skin side of the duck breasts in a diamond pattern.
In a dry skillet, toast the juniper berries over moderately high heat until they are shiny, about 1 1/2 minutes, being careful not to burn them. Immediately transfer the juniper berries to a mortar or spice grinder and let cool completely. Add the nutmeg, cinnamon, ginger, cloves, pepper and 1/2 teaspoon of salt to the mortar and grind to a fine powder. Transfer the spices to a small bowl and stir in the Armagnac and mustard to make a paste.
Rub the spice paste all over the duck breast halves. Set the duck breasts in a glass or ceramic dish, cover with plastic wrap and refrigerate for at least 6 and up to 24 hours. Remove the duck breasts from the refrigerator at least 30 minutes before cooking.
Preheat the oven to 325°. Set a heavy medium skillet over moderately high heat until very hot. Set the duck breasts in the pan, skin side down, and cook until the skin is browned and the fat begins to melt, about 3 minutes. Adjust the heat as necessary if the skin is browning too fast. Remove the skillet from the heat and transfer the breasts to a plate. Drain the fat and carefully wipe out the pan with paper towels. Return the duck breasts to the skillet, meat side down, and cook over moderately high heat for 5 minutes longer. Transfer the duck to a small baking dish and keep warm in the oven while you prepare the sauce.
Add the red wine and the vinegar to the skillet and bring to a simmer, stirring with a wooden spoon to release all of the browned bits from the bottom of the pan. Cook the sauce until reduced to 3 tablespoons. Remove from the heat and swirl in the butter. Season the sauce with salt and pepper.
Slice the duck breasts on the diagonal and fan them out on warmed plates or a warmed platter. Spoon the sauce over the duck and serve immediately.