In a small bowl, mix together miso, sake, mirin, soy sauce, and sesame oil.
Put the salmon fillet in a bigger bowl (skin side up). Then, pour the marinade over the salmon. Cover with plastic wrap. Refrigerate for 30 minutes.
Preheat oven to broil. Line a baking sheet with parchment paper.
Remove the salmon from the refrigerator. Remove the excess marinade from the salmon (otherwise it will quickly burn). Place the marinated salmon fillet on the prepared baking sheet, skin side down
Place the baking sheet at the middle rack. Broil for 10-12 minutes, or until the salmon fillet pieces are cooked.
Sprinkle the miso salmon with black and white toasted sesame seeds and chopped scallions.