Place the steak in the freezer for 20 to 30 minutes. This step helps make the beef firm enough to slice thin.
In a small bowl combine the broth (or water), brown sugar, soy sauce and sesame oil. Whisk and set aside.
Pat the beef dry with paper towels. Cut the beef against the grain into ⅛ inch thick slices. Add the beef to a bowl and toss with the cornstarch. Set aside.
Line a baking sheet with a few paper towels or a wire rack to drain the beef. Set aside.
Heat the oil in a large Dutch oven or deep cast iron skillet over medium-high until it reaches 375°F. Add one-fourth of the beef slices and fry until crispy around the edges, about 5 minutes. Stir occasionally to prevent the beef slices from sticking together. Monitor the temperature and adjust the burner as needed to maintain a minimum of 350°F to 375°F. Using a slotted spoon remove the beef to the prepared baking pan to drain. Repeat until all beef is cooked.
Transfer 1 tablespoon of the frying oil to a large nonstick skillet or wok. Heat on medium-high until shimmering. Add the white parts of the scallions, chiles, garlic and ginger. Cook, stirring constantly until fragrant, about 1 minute. Carefully add the sugar and soy sauce mixture and bring to a boil. Reduce the heat and simmer, stirring occasionally, until the sauce is slightly thickened and syrupy, about 5-7 minutes.
Add the beef and cook tossing constantly until the sauce coats each slice. Add the scallion greens and stir to combine. Continue to cook for 4-5 minutes.
Remove from the heat and serve immediately over cooked rice. Garnish with extra scallion greens and a few toasted sesame seeds if desired.