Place beef in a ziploc bag. Combine orange juice, soy sauce, ginger, garlic powder and cayenne pepper. Reserve half of the marinade; pour remaining marinade over steak. Marinate in refrigerator for 15 minutes.
While meat is marinating, toast sesame seeds in a dry skillet over medium-high heat, stirring frequently, until seeds are golden-brown and fragrant. Pour seeds out of the hot pan to avoid overtoasting. Set aside.
Heat oil in a large skillet over high heat. Add beef and 3 teaspoons toasted sesame seeds; saute about 3 minutes or until beef is browned. Remove from skillet; keep warm. Add vegetables to skillet; cook, stirring, until crisp-tender, about 3 to 5 minutes.
Combine reserved marinade with cornstarch and sugar. Add to skillet; bring to a boil. Return beef to skillet. Stir and heat through. Top with remaining 1 teaspoon toasted sesame seeds. Serve over rice, if desired.