Miso Black Cod (Nobu-Style)

0 Make the Marinade: In a pot, boil the mirin and sake for 3 minutes. Stir in the cane sugar and miso and cook a few minutes more, without letting it boil. Take it off the heat and cool completely.
1 Marinate the Cod: Spoon a little of the cooled marinade across the bottom of a non-reactive dish. Lay the cod skin side down, pour the rest over the top, and turn the fillets so every side is coated. Leave them skin side up, cover tightly with plastic wrap, and refrigerate at least 24 hours, up to 2 to 3 days.
2 Broil: Set the oven to broil on high (about 550°F/288°C). Line a baking sheet with foil and lay the cod on it skin side down. Broil on the middle rack about 7 minutes, adjusting for thickness, until the top caramelizes and the fish flakes. Check the thickest part with an instant-read thermometer: 135°F (57°C) for moist and just flaking, or 145°F (63°C) for fully cooked.
3 Optional Sauce: Pour the leftover marinade into a saucepan and boil at least 5 minutes, stirring often, since it touched the raw fish. Spoon over the cod before serving.