1. In a small saucepan, combine the miso paste, sake, mirin, sugar, and soy sauce. Heat over medium heat, stirring constantly, until the sugar is dissolved and the mixture is smooth. Let it cool to room temperature.
2. Place the black cod fillets in a shallow dish and pour the cooled marinade over the top, ensuring each piece is well-coated. Cover and refrigerate for at least 2 hours, or overnight for deeper flavor.
3. Preheat your oven to 400°F. Line a baking sheet with parchment paper to prevent sticking.
4. Remove the fillets from the marinade, allowing excess to drip off. Place them on the prepared baking sheet.
5. Bake for 10-12 minutes, depending on the thickness of the fillets, until the fish is opaque and flakes easily with a fork. For a caramelized finish, you can broil the fish for an additional 1-2 minutes.
6. Serve the black cod immediately, garnished with fresh herbs like cilantro or green onions if desired.

As someone who learned to cook through trial and error, I assure you these steps are straightforward and designed to make you feel at ease in the kitchen.