Combine rice vinegar, hoisin sauce, water, tamari, 2 teaspoons cornstarch, sesame seeds, and ground white pepper in a small bowl; set aside.
In a mixing bowl, toss diced tofu with dark soy sauce, sesame oil, salt, and the remaining cornstarch.
In a heated skillet or wok over medium heat, pour the oil and sauté minced garlic and grated ginger for a minute.