Put the cucumber in a medium bowl. In a small bowl, stir together the hot water, vinegar, sugar, and salt. Pour over the cucumber, making sure to submerge as much cucumber as possible. Set aside for at least 15 minutes (though ideally for like 3 days).

Put the zucchini in a large bowl. Bring a pot of water to a boil, and add the soba noodles and boil for 3-4 minutes (unless your package directions tell you otherwise) until tender but with a little bite. Drain, then, while hot, add to the zucchini and toss. Add as many cucumbers as you like, along with 2 tablespoons of the vinegar brine. Add the soy sauce and toss to coat. Add the scallions and cilantro, and toss. Taste, and add more vinegar, soy sauce, or salt as necessary. Sprinkle with the sesame seeds if using, and serve warm.