Combine soy sauce and garlic in a large zip-top heavy-duty plastic bag; add salmon steaks.
Seal bag, and marinate steaks in refrigerator up to 4 hours, turning occasionally.
Combine brown sugar and next 3 ingredients in a small saucepan; cook over medium heat, stirring until sugar dissolves.
Remove salmon steaks from marinade; discard marinade.
Brush brown sugar mixture on both sides of each steak.
Coat grill rack with cooking spray; place on grill over medium-hot coals.
Place salmon steaks'on rack, and cook 5 to 6 minutes on each side or until fish flakes easily when tested with a fork.
Garnish with fresh basil, if desired.