For a richer flavor, marinate the cubed tofu in a tablespoon of shoyu for about 30 minutes before adding it to the salad. Toasting the sesame seeds enhances their nutty flavor. Toast them in a dry pan over medium heat for a few minutes, until fragrant and lightly golden. If you don't have fresh ginger, 1/4 teaspoon of ground ginger can be used as a substitute. Feel free to adjust the amount of shoyu and sugar in the dressing to suit your taste preferences. Start with the recommended amount and add more to taste. This salad is best served fresh. If preparing ahead of time, keep the dressing separate and toss just before serving to prevent the watercress from wilting.