Whisk together the soy sauce, muscovado sugar, minced ginger, olive oil, and garlic in a bowl, and pour into a resealable plastic zipper bag. Add the salmon fillet, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator overnight or up to 2 days.
About 1/2 hour before grilling, soak cedar plank in water.
Preheat an outdoor grill for medium heat, and lightly oil the grate.
Remove the salmon from the marinade, and shake off excess. Discard the remaining marinade. Place the salmon, skin side down, onto the cedar plank.
Grill with the grill cover closed until the salmon is opaque but still juicy, about 20 minutes. Carefully remove the salmon from the plank in one piece, leaving the skin on the plank.