In a large bowl, combine all marinade ingredients: calamansi juice, lemongrass, soy sauce, minced garlic, ginger, peppercorns, and brown sugar. Mix well.

Add the chicken pieces to the marinade, ensuring they are fully submerged. Cover the bowl and refrigerate for at least 4 hours, or preferably overnight for best flavor penetration.

Prepare the basting oil: Gently heat the vegetable oil in a small saucepan. Add the annatto seeds and smashed garlic. Simmer on low heat for 5-7 minutes until the oil turns a deep orange-red. Strain the oil, discarding the solids, and stir in the fish sauce and butter (if using). Set aside.

Preheat your grill to medium-high heat. Remove the chicken from the marinade, allowing excess liquid to drip off (discard the remaining marinade). Lightly brush the chicken with the annatto basting oil.

Grill the chicken over direct heat for 8-10 minutes per side, turning occasionally, until cooked through (internal temperature reaches 165°F or 74°C). During the last 5 minutes of grilling, frequently brush both sides with the basting oil to develop a nice glaze.

Remove the Inasal from the grill. Let it rest for 5 minutes before serving hot, traditionally alongside steamed rice and an extra dipping sauce of calamansi juice mixed with fish sauce.