In a medium bowl, whisk together the white miso paste, mirin, sake (or dry sherry), granulated sugar, and optional soy sauce and sesame oil until smooth.

Cut the salmon into 4-6 individual fillets, roughly 6 oz (170g) each. Place the fillets in a shallow dish or a resealable bag.

Pour the miso marinade over the salmon, ensuring all pieces are well coated. Cover and refrigerate for 1 to 2 hours. Do not marinate thin fillets overnight due to the saltiness of miso.

Preheat your broiler to high. Line a baking sheet with aluminum foil and lightly grease it with vegetable oil to prevent sticking.

Carefully remove the salmon from the marinade. Using an offset spatula or butter knife, gently scrape off any excess marinade from all sides of the fillets. This prevents the marinade from burning under the high heat of the broiler.

Place the salmon fillets, skin side down (or flesh side down if skinless), on the prepared baking sheet, leaving some space between each piece.

Broil the salmon for 8 to 12 minutes, depending on the thickness of your fillets, until the top is beautifully golden brown and caramelized, and the fish flakes easily with a fork. To ensure a perfectly juicy salmon, aim for an internal temperature of 135-140°F (57-60°C) with an instant-read thermometer, as carryover cooking will bring it to the recommended 145°F (63°C).

Carefully remove the salmon from the baking sheet and serve immediately. Garnish with sesame seeds or chopped green onions if desired.