Whisk together miso, sake, mirin, soy sauce, oil, and sugar. Rub mixture over every surface of fish fillets. Transfer to a plastic zipper-lock bag or sealable container. Marinate for about 20-30 minutes.

Adjust broiler rack to 4 inches from heat source and preheat to high. Cover a small broiler pan with aluminum foil. Place black cod fillets skin side down on pan. Broil until top surface is well charred and a thin skewer inserted into black cod shows no resistance at all when piercing through layers of flesh, about 10-12 minutes.