Over a double boiler, combine miso, sugar, sake, and mirin and whisk until sugar dissolves. Remove from heat and let cool. Refrigerate for a few hours so that marinade is cold and add black cod fillets or fillet pieces. Marinate for 24 to 48 hours in refrigerator.
Remove from marinade and scrape off most of marinade that is coating the fish. Preheat oven to broil (oven should be very hot and heat should be coming from the top down). Place fish on a greased foil-lined baking sheet. Place baking sheet approximately 6 inches away from top heating element. Broil 7 to 10 minutes, until browned. Depending on the thickness of the fish, fish should flake away; otherwise, bake an additional few minutes.