Blanch broccoli by bringing a large pot of water to a boil. Once boiling, add the broccoli and cook for 1-2 minutes. Place in cold water, then drain and dry.
While water is coming to a boil, whisk together 2 tbsp cornstarch with 2 tbsp water in a large bowl. Add beef and toss in the mixture.
In another bowl, whisk the remaining cornstarch (1 tbsp) with soy sauce, rice vinegar, brown sugar, garlic, sesame oil and ginger - set aside
Add 1 tbsp vegetable oil to a large pan and cook on medium heat. Add meat and cook until heated through and pink is gone. Take out meat and set aside
Once broccoli is blanched and done, add 1 tbsp vegetable oil to the pan and cook broccoli for about 2 minutes. Add the beef and the sauce to the pan and bring to a boil. Cook for 1-2 minutes and serve over rice or noodles sprinkled with sesame seeds.