Grind white peppercorns and coriander seeds into a powder using a mortar and pestle or a coffee grinder.
Add lemongrass and garlic and pound into a fine paste.
Transfer into a bowl and add all remaining marinade ingredients; stir to dissolve the sugar.
Split the breasts in half so you have two split, bone-in breasts and separate the drums from the thighs.
Trim off excess skin from the thighs, if any.
Add chicken pieces to a zip-top bag or a casserole dish and pour the marinade over, making sure all the pieces are well coated.
Press out as much air as possible and marinade for at least 2 hours or overnight, stir the chicken or flip the bag half way through to ensure even marinating.
Pull the chicken out to room temperature about 45 minutes before cooking.
Preheat the grill on medium heat, then place the chicken on the grill, skin-side up.
Turn the heat down to low or medium-low and cook with the lid closed, flipping every 5 minutes, until done, about 18-30 minutes depending on the size of the pieces.
Cook the chicken breast until the internal temperature reaches 160 °F or 72°C, and cook chicken drums and thighs until internal temp of 175°F (80°C).
Let the chicken rest for 10-15 minutes before serving.
