Bring a large pot of water to a rapid boil. Add the soba noodles and cook according to the package directions, usually 4 to 6 minutes, stirring once or twice so they don't clump. Start testing a minute early: the noodles are done when they are tender with a slight chew (al dente).

While the noodles cook, make the sauce. In a small bowl, whisk together the dark sesame oil, 2 tablespoons of the soy sauce or tamari, tahini (if using), agave nectar, ginger, rice vinegar, and sesame seeds until smooth.

Drain the noodles in a colander. If you plan to serve them cold or at room temperature, rinse them under cold running water and drain well, which removes the surface starch and keeps the noodles from sticking. For a warm dish, simply drain and shake off the excess water. Transfer the noodles to a large serving bowl.

Drizzle the sauce over the noodles and toss with tongs or two forks until every strand is evenly coated. Taste, and add the remaining tablespoon of soy sauce if you'd like it saltier.

Serve warm or at room temperature, passing extra sesame seeds at the table for topping individual servings.