Stir first 4 ingredients in small bowl until sugar dissolves. Set sauce aside. Place beef in large bowl; sprinkle with salt and pepper. Add cornstarch and toss to coat.
Heat 1 1/2 tablespoons oil in large wok over high heat. beef mixture; stir-fry until no longer pink outside, about 3 minutes. Transfer beef mixture to medium bowl. Heat remaining 1 tablespoon oil in same skillet. Add broccoli, yam, and ginger.
Toss to coat; sprinkle with salt and pepper.Cover, reduce heat to medium-high, and cook until vegetables are just tender, about 5 minutes. Add beef mixture. Toss until sauce coats beef, about 1 minute.