Slice the flank steak into thin, bite-sized pieces. Toss with cornstarch, kosher salt, and black pepper until well-coated. Let sit for about 10 minutes.

In a saucepan over medium heat, combine soy sauce, chicken broth, dark brown sugar, ginger, and garlic. Cook for about 15 minutes, or until slightly thickened.

Heat about 1 inch of vegetable oil in a wok or large skillet over medium-high heat. Fry the coated beef in batches until golden and crispy, about 1-2 minutes per side.

Remove excess oil from the wok. Add remaining garlic and ginger, sauté for 2 minutes. Pour in prepared sauce and boil gently for 5 minutes.

Add crispy beef to the sauce, tossing gently to coat evenly and cook for 1 more minute.

Transfer to a serving platter, garnish with green onions and sesame seeds, and serve over white rice or with vegetables.