Toast 2 teaspoons of coriander seeds, then grind with 1½ teaspoons of white peppercorns. Pound in 1 lemongrass stalk, bottom half only, and 5 garlic cloves.
Stir in 3 tablespoons of soy sauce, 3 tablespoons of fish sauce, 1 tablespoon of black soy sauce, 1 tablespoon of sugar, and ½ cup of water until the sugar dissolves.
Coat 8 bone-in, skin-on chicken thighs in the marinade and refrigerate for at least 2 hours. Take them out 45 minutes before cooking.
Preheat a grill to medium. Grill the chicken skin-side up, then lower to medium-low, close the lid, and flip every 5 minutes for 18-30 minutes, until the thickest part reaches 175°F / 80°C.
