This recipe is best when the meat is sliced paper-thin. Freezing the steak for 30-45 minutes prior to slicing makes the task easier.
Trim and discard any fat from flank steak. Slice the meat horizontally (with the grain) Into three pieces. Cut each piece (against the grain) into very thin slices. In a bowl, combine the soy sauce, cornstarch, sugar and cayenne. Add the sliced flank steak and marinate, refrigerated, for 30-45 minutes.
Heat oil (just to coat bottom of large skillet) over high heat. Add broccoli and stir-fry 2-3 minutes. Using a slotted spoon, remove broccoli to serving bowl. Add flank steak and marinade to pan (add more oil if necessary) and, stirring constantly, cook 3-4 minutes or until beef is lightly browned and glazed. Return broccoli to pan and heat through. 4 servings.