Spray a skillet with cooking spray, and place over medium heat. Cook and stir the chicken until lightly browned and no longer pink, about 5 minutes. With a spatula, spread the chicken pieces out into the bottom of the skillet. Spray again with cooking spray if needed.
In a bowl, beat the egg whites with soy sauce, and stir in the bean sprouts, water chestnuts, and green onion. Pour the egg mixture into the skillet over the chicken, and let cook until the bottom is lightly golden and the top has nearly set, about 3 minutes. Flip the egg pancake over, and cook the other side until lightly golden and set, another minute or two.