Place the salmon fillets, soy sauce, and mirin in a large, resealable plastic bag. Seal the bag, gently turning several times to coat the fish with marinade, and refrigerate for 30 minutes.
Place an oven rack on the center shelf. Preheat the oven to 425 degrees F, and lightly grease a baking sheet with cooking spray or olive oil. Remove the fish from the marinade, patting dry with paper towels. Reserve 3/4 cup of the marinade, discarding the remainder.
Place the fish skin-side down on the prepared baking sheet. Cook for 11-13 minutes, until just cooked through (for tender results, it’s better to slightly undercook than to overcook salmon).
While the salmon is baking, prepare the sauce. Place the cornstarch in a small saucepan and whisk in 2 tablespoons of the reserved marinade to create a slurry. Whisk in the remaining marinade, garlic, ginger and both sugars. Turn the heat to medium and continue whisking frequently until the sauce thickens, approximately 5 minutes. Remove from the heat and whisk in the sesame oil.
Serve the salmon over rice (or with grains/vegetables of your choice) and top with the teriyaki sauce. Garnish with sliced scallions.
Mirin is a Japanese wine similar to sake, but it has more sugar and a lower alcohol content. You can find mirin in the international aisles of most grocery stores or purchase it online. Some brands like Kikkoman label it as aji-mirin.You can substitute in a dry sherry or sweet marsala wine for mirin. Rice vinegar or dry white wine will also work, but you'll need to balance the sourness by adding around 1/2 teaspoon sugar per tablespoon.