Put the squid in a bowl with the sake and turn it over to coat. Leave for a minute or two, then drain thoroughly and pat dry. Wet squid steams instead of searing and goes tough.
Cut the spring onion diagonally into 5 mm pieces and tear the maitake into bite-size clusters. Mix the soy sauce and mirin together in a small bowl now - there is no time to measure once the pan is hot.
Heat the oil in a frying pan over medium heat and cook the spring onion until it softens and colors at the edges. Add half the butter (about 10 g) and the maitake, and stir-fry about 1 minute until the mushrooms start to brown.
Push the mushrooms and onion to one side of the pan. Add the remaining butter to the empty side and put the squid in there. The mushrooms need longer than the squid, which is why they are kept separate.
Cook the squid until it just turns from translucent to opaque white - under a minute on a hot pan. The rings will curl slightly. Do not cook it further.
Pour the soy sauce and mirin over everything, turn the heat to high, and toss for about 30 seconds to drive off the liquid and let the soy sauce brown in the butter. Take the pan off while it still looks glossy rather than dry.
Plate and sprinkle ichimi togarashi over the top. Serve immediately.
