Place the wakame in a medium bowl and add enough cold water to cover it by 2 inches, then let it stand for about 10 minutes until softened. The seaweed should expand noticeably and feel flexible but still a little springy.

Drain the wakame in a colander, squeeze out as much water as possible with your hands, then press it again between paper towels. When it's dry enough, the seaweed will clump together tightly instead of dripping into the sink.

In another medium bowl, whisk together the soy sauce, rice vinegar, mirin, toasted sesame oil, and white miso, then season with salt and pepper. Keep whisking until the miso fully dissolves and the dressing looks smooth, glossy, and evenly blended.

Add the dried wakame to the bowl of dressing, season again with salt and pepper, and toss until every strand is coated, breaking up any clumps as you go. The seaweed should look lightly glossy and evenly seasoned, with no dry patches hiding in the middle.

Transfer the salad to a serving bowl and finish with sesame seeds or scallions, if using, then serve at room temperature or chilled. The flavor tastes a little rounder after a short chill, but the texture should still stay snappy.