Step 1: Preparing the Miso Marinade: In a small saucepan, bring the sake and mirin to a boil over medium-high heat. Let it boil for about 20 seconds to evaporate the alcohol. Turn the heat down to low and add the miso paste. Whisk until the miso has dissolved completely. Increase the heat slightly and add the sugar, whisking constantly until the sugar is fully dissolved. Remove from heat and let the marinade cool completely to room temperature before using.
Step 2: Marinating the Fish: Pat the black cod fillets dry with paper towels to ensure the marinade sticks well. Place the fish in a non-reactive dish or a large resealable plastic bag. Pour the cooled miso marinade over the fillets, ensuring they are thoroughly coated on all sides. Cover the dish or seal the bag and refrigerate for at least 24 hours. For the best results, let it marinate for 2 to 3 days to allow the sugars and salt to cure the fish.
Step 3: Searing the Fillets: Preheat your oven to 400°F (200°C). Remove the fish from the refrigerator and gently wipe off the excess marinade with your fingers—do not rinse the fish with water. Heat a bit of vegetable oil in an oven-safe skillet over medium-high heat. Once the pan is hot, place the cod skin-side up in the pan and sear for 2 to 3 minutes until the surface is deeply browned and caramelized.
Step 4: Baking to Perfection: Carefully flip the fillets over so they are skin-side down. Transfer the entire skillet to the preheated oven. Bake for about 5 to 10 minutes, depending on the thickness of the fish. The cod is done when it flakes easily with a fork and the center is opaque. If the top isn’t dark enough, you can place it under the broiler for the final 60 seconds.