Preheat oven to 450°F. Line bottom of a broiler pan with foil, then oil rack of pan. Boil soy sauce and maple syrup in a small saucepan over moderate heat until glaze is reduced to 1/3 cup, about 5 minutes. Arrange salmon, skin side down, on rack of broiler pan and pat dry. Reserve 1 1/2 tablespoons glaze in a small bowl for brushing after broiling. Brush salmon generously with some of remaining glaze. Let stand 5 minutes, then brush with more glaze. Roast salmon in middle of oven 10 minutes. Turn on broiler and brush salmon with glaze again, then broil 4 to 5 inches from heat until just cooked through, 3 to 5 minutes. Transfer salmon with 2 wide metal spatulas to a platter, then brush with reserved glaze using a clean brush.