Place the chicken, breast side down, on a cutting board. Using a large, sharp knife or cooking shears, cut all the way down one side of the backbone; open the chicken and cut a slit in the cartilage at the back of the breastbone. Apply pressure to expose the dark bone, but do not remove. Cut along the other side of the backbone; remove. Lay the chicken flat in a glass baking dish.
In a medium bowl, whisk the sugar and 1/2 cup hot water until the sugar is dissolved. Whisk in the fish sauce, lime juice, chile and half of the garlic. Transfer 1/2 cup of the fish sauce mixture to a small bowl; refrigerate.
Stir 1/3 cup water, the ginger, soy sauce and remaining garlic into the remaining sauce; pour over the chicken to marinate. Cover and refrigerate for at least 3 hours or up to 10 hours. Remove and let stand at room temperature for 45 minutes before grilling. Remove the reserved sauce from the fridge.
Meanwhile, preheat a grill to between 400 degrees and 425 degrees ; light one side to high or medium-high; keep the other side unlit. Transfer the chicken to the cool side of the grill, cover and cook, without turning, until the skin is golden-brown and the juices run clear when the thigh is pricked with a sharp knife, about 55 minutes. (To crisp the skin, turn the flame under the chicken to low and turn the bird over for the last 5 minutes of cooking.) Transfer to a cutting board; let stand for 10 minutes. Slice or cut into pieces and serve with the reserved sauce for dipping.