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 barbecuing, soak cedar plank in water.
Preheat an outdoor barbecue 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.
Barbecue with the barbecue 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.
Cedar planks for barbecuing can be purchased online or in specialty shops.