Step 1: In a small saucepan, bring the sake and mirin to a boil over high heat. Let it boil for 20 seconds to evaporate the alcohol.
Step 2: Turn the heat down to low and add the miso paste and sugar. Whisk constantly until the sugar has dissolved and the marinade is smooth. Remove from heat and let it cool to room temperature.
Step 3: Pat the black cod fillets very dry with paper towels. Place the fish in a non-reactive container or a glass storage dish and pour the marinade over it, ensuring the fish is fully submerged.
Step 4: Cover and refrigerate for 2 to 3 days. This long marination time is essential for the easy miso black cod nobu flavor.
Step 5: When ready to cook, preheat your oven broiler. Lightly wipe off excess marinade from the fish using your fingers. Place the fillets on a foil-lined baking sheet.
Step 6: Broil the fish for 5-8 minutes, or until the top is charred and caramelized and the fish flakes easily with a fork. Serve immediately.