To make Prune stuffing, in large skillet, heat butter until hot. Add shallots; cook over medium heat 4-6 minutes or until translucent. Add prunes and Armagnac; cook over medium-high heat 1 minute, stirring occasionally. Process in food processor container until prunes are chopped, pulsing on and off; set aside.
Using long thin knife, cut a 3 to 4-inch wide pocket through the roast, cutting from one end of roast to the opposite end. Do not cut through sides of roast, leaving at least 3/4-inch of roast uncut on either side of pocket. Spoon Prune Stuffing into pocket. Place 3 to 4-inch square piece of double layer aluminum foil to cover filling at each end. Using kitchen twine, tie around length of roast several times to firmly secure foil pieces during cooking.
Place roast, fat side up, on rack in broiling pan. Season surface of roast with salt and pepper. Insert ovenproof meat thermometer so tip is centered in thickest part of roast. Do not add water or cover. Roast at 350 degrees F for 55 to 65 minutes or until meat thermometer registers 155 to 160 degrees for medium doneness.
Meanwhile prepare couscous according to package directions, using chicken broth in place of water and salt.
Remove roast to carving board; tent with foil and let stand 10 minutes. Remove twine and foil; carve crosswise into 6 slices. Season with additional salt and pepper, if desired. Serve with prepared couscous.
Makes 6 servings.