In a small bowl combine ingredients for the marinade; soy sauce, rice wine vinegar, sugar, salt and cornstarch.
Pour marinade over strips of beef. Massage marinade into the beef. Allow beef to marinate for 15-20 minutes.
In another small bowl combine the ingredients for the stir fry sauce; soy sauce, rice wine vinegar, sesame oil, sugar, salt, garlic powder, and red pepper flakes. Stir to combine. Set aside.
In a small dish combine cornstarch and water to make the thickener. Set aside.
Heat oil in a wok or large skillet.
Add marinated beef strips to the wok, and cook stirring frequently over medium high heat until beef is heated through, 2-3 minutes.
Add broccoli and stir to combine with the beef strips. Cook until crisp tender, about 2 minutes.
Add stir fry sauce and stir to combine.
Add cornstarch slurry and cook until sauce thickens and becomes glossy.
Serve immediately over hot seamed rice.
Top with a sprinkle of sesame seeds to serve.