In a medium saucepan over low heat, combine ¾ cup white miso paste, ½ cup sake, ½ cup mirin, and ¼ cup sugar. Stir continuously until the sugar is fully dissolved, which should take about 5 minutes. Remove from heat and let cool to room temperature.

Place the 4 pieces of black cod in a ziplock bag or shallow dish. Pour the cooled marinade over the fish, ensuring each piece is well-coated. Seal the bag or cover the dish tightly and refrigerate for 48-72 hours.

Preheat your oven to 450°F (230°C). Line your baking sheet with parchment paper.

Carefully remove the black cod from the marinade, gently wiping off any excess with a paper towel. Place the fish fillets skin-side down on the prepared baking sheet.

Bake the black cod for 10-12 minutes until opaque and flakes easily with a fork. Broil for an additional 1-2 minutes for caramelization.

Garnish with freshly chopped scallions and serve immediately.