Remove the strings from the celery. Slice the stalk diagonally into bite-sized pieces and chop the leaves roughly. Trim the base of the maitake mushrooms and separate them by hand.

Defrost the squid and pat it dry. Place it in a microwave-safe container and rub with salt koji and black pepper.

Add the celery stalks, maitake mushrooms, butter, and soy sauce to the container. Cover loosely with plastic wrap and microwave at 600W for 3 minutes and 30 seconds.