Rinse off the black cod with sake. Pat dry. Placing the skin side down, pour salt evenly over top of the fillet.Let sit for 30 minutes to draw out excess moisture.
To Make Miso Marinade:
In a medium saucepan combine mirin and sake. Cook on medium heat until the mixture comes to a boil. Let it boil for 20 seconds. Reduce the heat and add miso paste. Stir on low heat, until the miso paste is fully dissolved. Remove miso marinade from the heat and let cool.
To Prepare Black Cod Fillet:
After 30minutes, brush all the salt off. Soak a paper towel or cheese cloth with sake.Use the wet sake cloth to wipe away any remaining salt. Place fillet in a large airtight container. Fillet can also be stored in a large zip lock baggie that is left to rest flat in a large backing dish. Pour in the miso marinade. Gently mix around the fillet, making sure to coat the entire fish. Leave the skin side facing up. Make sure the entire fillet is resting flat and is not folded or crimped together.
Marinate for 3 days.
To Cook the Black Cod:
Preheat oven to 425 F. Remove Black Cod from the marinade. Brush off as much of the miso as possible. Lightly grease a glass baking dish. Place fillet in baking dish, skin side up. Cook for 15-20 minutes.Put oven on broil. Broil for 30-60 seconds to finish blistering the skin. The skin should be blistered and lightly burnt around the edges. Remove from the oven and let rest for 5 minutes. Cut fillet into 4 even portions and serve immediately.