Begin by making the miso marinade: Combine the mirin and sake in a small saucepan and bring to a gentle boil for about 30 seconds. Reduce the heat to low and whisk in the miso paste and then the sugar to fully combine and dissolve the sugar. Transfer to a metal bowl to cool.
Dry the salmon with paper towels, but don't rinse. Place the fillets skin-side up in a shallow-rimmed dish and pour on the marinade so it touches all sides of the fish. Cover tightly with plastic wrap and refrigerate for 1 to 3 days.
To cook: Preheat the broiler. Scrape off excess marinade and place the salmon fillets skin-side down on a parchment-lined baking sheet. Broil for 8 to 15 minutes, or until the internal temperature reaches 125 to 130° F for medium or 140° F for well-done.
Garnish with toasted sesame seeds and sliced scallions. Serve immediately.
The longer the salmon marinates, the stronger the flavor of the marinade will be.
Start with the freshest salmon you can find because of the long marinating time.
You can flip the salmon halfway through marinating if it isn't completely covered.
The size, thickness and type of salmon change the cooking time. Wild salmon will cook more quickly than farmed salmon. Verify temperatures with an instant-read probe thermometer.
All broilers are different, so watch your fish closely as it cooks.
It can also be seared on a grill and finished in a 400° F oven.