Combine sauce ingredients in small bowl & stir thoroughly.
Cut steak across the grain into thin slices, about 1/2" deep by 2" long. In bowl big enough to hold meat, combine egg whites, salt & water chestnut flour. Beat with chopstick until frothy.
Add steak, & use fingers to coat each slice. Deep-frying: In wok, heat oil to moderately hot. When ready, piece of coated meat will rise to surface immediately. 2
Add beef all at once, & toss with sauce until beef is hot & coated. Push beef out of sauce, dribble in cornstarch paste to lightly thicken.
Add beef all at once, & toss with sauce until beef is hot & coated. Push beef out of sauce, dribble in cornstarch paste to lightly thicken.