Preheat oven to 375 degrees F.
Place duck skin-side down on a cutting board. Trim off all excess skin that hangs over the sides. Turn over and make three parallel, diagonal cuts in the skin of each breast, cutting through the fat but not into the meat. Sprinkle both sides with five-spice powder and salt.
Place the duck skin-side down in an ovenproof skillet over medium-low heat. Cook until the fat is melted and the skin is golden brown, about 10 minutes. Transfer the duck to a plate; pour off all the fat from the pan. Return the duck to the pan skin-side up and transfer to the oven.
Roast the duck for 10 to 15 minutes for medium, depending on the size of the breast, until a thermometer inserted into the thickest part registers 150 degrees F. Transfer to a cutting board; let rest for 5 minutes.
Pour off any fat remaining in the pan (take care, the handle will still be hot); place the pan over medium-high heat and add orange juice and honey. Bring to a simmer, stirring to scrape up any browned bits. Add orange zest and soy sauce and continue to cook until the sauce is slightly reduced, about 1 minute. Stir cornstarch mixture then whisk into the sauce; cook, stirring, until slightly thickened, 1 minute. Remove the duck skin and thinly slice the breast meat. Drizzle with the orange sauce.
Ingredient noteBoneless duck breast halves range widely in weight, from about 1/2 to 1 pound, depending on the breed of duck. They can be found in most supermarkets in the poultry or specialty-meat sections or online at mapleleaffarms.com or dartagnan.com.Roasting Tips1. Very cold meat won't roast evenly. Place it on the counter while preheating the oven.2. Durable cotton kitchen string is sold at kitchenware stores, most gourmet markets and large supermarkets. Do not use sewing thread or yarn, which may contain inedible dyes or unsavory chemicals.3. A heavy-duty, high-sided roasting pan is essential for conducting heat evenly. Never substitute a cookie sheet. A broiler pan will work in a pinch, but the roast will inevitably be somewhat chewier.4. Give it a rest. A roast's internal temperature will rise about 10 degrees while resting. The natural juices will also reincorporate into the meat's fibers and the skin or crust will dry out slightly for a more toothsome yet more succulent dinner.