If you haven't read through the full article above, do so now. Plan this cook at least a day in advance.
Pat the black cod fish fillets dry with paper towels. Use tweezers to remove any pin bones.
Slice the fillets into smaller pieces, about 3-4 inches long and wide. Place them in a shallow dish and set the fillets aside.
In a pot, combine the sake, mirin, and miso. Bring the mixture to a boil and allow it to simmer for 5 minutes. Remove from the heat and cool
Let it cool fully to room temperature and add the cool water, sugar, and salt.
Pour the mixture over the black cod fillets so they are fully covered over the top. Cover it with plastic wrap and place in the fridge to marinate for 4-8 hours.
Remove from the marinade and rinse each piece of cod. Pat each piece very dry with paper towels.
Place each piece of cod onto a wire rack over a cookie sheet. Place the rack of fish back into the fridge and leave it uncovered overnight to form the pellicle.