Make the miso marinade: In a mixing bowl, whisk together the miso paste, mirin, sake, sugar, soy sauce, and grated ginger (if using) until smooth and the sugar has dissolved completely.
Prepare the fish: Pat the black cod fillets dry with paper towels. Check for any pin bones and remove them with fish tweezers or pliers if necessary.
Marinate the fish: Place the cod fillets in a shallow dish and pour the miso mixture over them, making sure each piece is thoroughly coated. Cover with plastic wrap and refrigerate for 24-48 hours (minimum 12 hours, but longer is better).
Preheat your oven: When ready to cook, set your oven to broil and position a rack about 6 inches from the heat source. Line a baking sheet with parchment paper.
Remove excess marinade: Gently wipe off the excess marinade from the fish fillets with your fingers, leaving just a thin coating. Too much marinade will burn under the broiler.
Broil the fish: Place the fillets skin-side down on the prepared baking sheet and broil for 8-10 minutes until the top is caramelized and the fish is opaque and flakes easily with a fork. The internal temperature should reach 135°F.
Rest briefly: Allow the fish to rest for 1-2 minutes before serving to let the juices redistribute.
Serve immediately: Garnish with sliced scallions and a sprinkle of sesame seeds. Serve with steamed rice and a simple vegetable side like steamed bok choy or a cucumber salad.