Trim the chicken of excess fat and cut each fillet in half (cut into thirds if large). Place in a large non-metallic dish or plastic bag.
Trim the coriander roots of the fine hairs then grind with the remaining ingredients to a paste using a mortar and pestle. Add the paste to the chicken and mix until the chicken is evenly coated. Cover and leave in the refrigerator for at least 15 minutes (longer is fine too).
To make the sauce, combine the vinegar and sugar in a small saucepan. Stir over low heat until the sugar is dissolved, then bring to a simmer, cooking uncovered for 5-10 minutes until the syrup starts to turn a caramel color and has reduced by approximately a third. Remove from the heat and cool for 5 minutes. Stir in the garlic and sambal oelek.
Cook the chicken on a preheated grill for 5-7 minutes on each side, basting with the coconut milk, until browned and cooked through. Serve with the sweet vinegar sauce.