In a small saucepan, heat the sake and simmer 30 seconds to cook off the alcohol. Whisk in the mirin and brown sugar until suger is dissolved. Whisk in the miso paste, whisking until completely smooth.
Transfer the marinade to a medium-sized baking dish and let cool (reserve 2 tablespoons of marinade in a separate small bowl if you want to make the miso aioli). Add the black cod fillets to a baking dish, pour the marinade over top, cover, and refrigerate 24 hours, or up to 3 days.
Preheat the oven to 400° (or use the broiler, see notes). (*See notes for skinless fish). Heat a little salted oil in a large oven-proof skillet over medium-high heat. Gently brush off any marinade from the skin side of the fish. Place the fish, skin side down, in the skillet and sear until golden and crispy, about 3 minutes. Spoon a little marinade over the top of the fish, season with salt, then place the fish in the oven to finish baking for 8-10 minutes, until flaky and golden.
Sprinkle with toasted sesame seeds and chives.
