Stir together the mirin and soy sauce in a small bowl. Brush a very light coating of soy and mirin on both sides of the sheets of nori and dehydrate the nori in a dehydrator for 1 hour.
Crumble the seaweed, transfer to a spice grinder and pulse on and off a few times: the nori powder should be a mix of flakes and dust. Stored in a covered container away from su