To prepare dressing: Combine rice wine (or sake), black bean sauce, soy sauce, sugar and water in a small bowl; stir until the sugar has dissolved. Set aside.
To prepare tofu & soba: Cut tofu into 8 pieces the size of a playing card.
Combine rice wine (or sake), soy sauce and 1 teaspoon sesame oil in a shallow dish just large enough to hold tofu in a single layer. Add tofu; cover and marinate in the refrigerator for 30 minutes, turning once.
Meanwhile, cook noodles in a large pot of boiling water until tender, about 5 minutes. Drain and rinse under cold running water. Place in a large bowl. Remove tofu from marinade and pat dry. Mix any marinade not absorbed by tofu into the reserved dressing and add to the noodles along with watercress, scallions and fermented black beans, if using; toss to combine. Set aside.
Spread sesame seeds on a small plate. Dip tofu into sesame seeds to coat one side. Heat remaining 1 teaspoon sesame oil in a large nonstick skillet over medium heat. Cook tofu, sesame-seed-side down (in batches, if necessary), until golden, about 1 minute. Turn and cook the other side until golden, 1 to 2 minutes more.
Divide the noodles among 4 plates and arrange sesame-crusted tofu on top. Serve immediately.
Note: Black bean-garlic sauce, a savory, salty sauce used in Chinese cooking, is made from fermented black soybeans, garlic and rice wine. It can be found in the Asian section of large supermarkets or at Asian markets. Use it in stir-fries and marinades for beef, chicken or tofu. Sake is a dry rice wine generally available where wines are sold. Junmai, a special designation for sake, denotes sake brewed from rice that has been milled less than other special-designation sakes. More pure than other sakes, junmai has no distilled alcohol added. It is characterized by a well-rounded, rich flavor and body and more acidity than most sakes. Fermented black beans, which are preserved in salt and flavored with ginger, are frequently used in Chinese stir-fries, marinades and sauces. Before using, soak them in water for 10-30 minutes to get rid of excess salt. When purchasing fermented black beans, look for shiny, firm beans, rather than ones that are dull and dry with salt spots. Look for them in Asian markets and natural-foods stores. Once open, store tightly wrapped in the refrigerator for up to 1 year.