In a small bowl, whisk together the miso, 4 teaspoons of honey, the soy sauce, mirin, sesame oil and cayenne. Measure out 2 tablespoons and brush onto the top and sides of the salmon fillets. Let stand at room temperature for 20 minutes.
Meanwhile, into the remaining miso mixture, whisk the remaining 1 teaspoon honey and 2 tablespoons water; set aside.
Heat the broiler with a rack about 6 inches from the broiler element. Mist a wire rack with cooking spray, then set in a rimmed baking sheet.
Evenly space the fillets, skin down, on the rack. Broil until the thickest part of the fillets reaches 110°F to 115°F, or are nearly opaque when cut into, 6 to 8 minutes.
Transfer to a serving platter and drizzle with about 2 tablespoons of the miso mixture, then sprinkle with sesame seeds and scallion. Serve with the remaining miso mixture.
