Soak the cedar plank in water for at least 1 hour.
In a small, non-reactive saucepan over medium–low heat on the stove, combine the soy sauce, vinegar and sugar. Cook until reduced by half, about 20 minutes. Remove from the heat and add the garlic, ginger and red pepper flakes. Cool slightly and then whisk in the oil. Reserve 120 millilitres of the glaze for basting the chicken.
Related Grill Skills
Put the thighs in a large bowl, pour in the glaze and toss to coat. Refrigerate until you are ready to grill.
Prepare the grill for indirect cooking over medium heat (180°C to 230°C).
Place the soaked plank over direct heat and close the lid. After 5 to 10 minutes, when the plank begins to smoke and char, turn the plank over.
Remove the thighs from the bowl and discard the glaze. Arrange the thighs on the plank and cook over direct heat, with the lid closed, for 5 to 10 minutes. Then move the plank over indirect medium heat and continue cooking, with the lid closed, until the juices run clear, 20 to 30 minutes, basting occasionally with the reserved glaze during the last 10 to 15 minutes of grilling time. Remove from the grill and baste with the glaze once more before serving.