Pound lemongrass, garlic, coriander root, and peppercorns in a mortar and pestle until you get a rough paste.

Transfer the paste to a mixing bowl. Add sugar, all sauces, and evaporated milk. Mix well until smooth.

Add chicken thighs and toss to coat evenly in the marinade. Cover and refrigerate for a few hours or overnight. Flip halfway through if not fully submerged.

Grill over low-medium heat, turning occasionally until the skin is golden and slightly charred and the meat is cooked through.

Serve with sticky rice and your favorite dipping sauce.