Fill a large pot with water and bring it to a boil over high heat.

Set a colander in the sink and put a large mixing bowl on the counter for dressing and tossing the noodles.

Put the peanut butter, soy sauce, rice vinegar, toasted sesame oil, and 2 tablespoons warm water into the mixing bowl.

Whisk the dressing until smooth. If it looks very thick, whisk in 1 more tablespoon warm water so it will coat the noodles easily.

Slice the cucumber thinly and set it aside on a plate.

Slice the scallions thinly. Keep a small handful of the green tops aside for the final garnish.

When the water reaches a full boil, add the soba noodles to the pot.

Stir the noodles right away so they do not stick together.

Cook the noodles for 4 to 5 minutes, until they are tender but still slightly springy in the center.

Drain the noodles in the colander.

Rinse the noodles under cold running water until they are completely cool and no longer feel starchy on the surface, about 30 seconds.

Shake the colander well so the noodles are not watery, then transfer the noodles to the bowl with the dressing.