Begin by crafting the marinade. Use a mortar and pestle to pound together the lemongrass, garlic, ginger, and salt until they form a coarse, aromatic paste and begin to release their natural oils.
Transfer the mixture to a bowl and stir in the annatto oil, cane vinegar, brown sugar, calamansi juice, liquid seasoning, and ground black pepper. Whisk until everything is thoroughly combined into a vibrant, fragrant marinade.
Place the chicken thighs in a large resealable plastic bag or non-metallic container. Pour in the marinade, making sure every piece is evenly coated. Seal and refrigerate for at least 1 hour, or preferably overnight for deeper flavor.
To cook, preheat your oven to 375°F (190°C). Arrange the marinated chicken pieces on a wire rack set over a baking sheet to allow even heat circulation. Bake for 45 to 50 minutes, or until the internal temperature of the chicken reaches 165°F (74°C).
While the chicken roasts, prepare the basting sauce. In a bowl, mix the melted butter with annatto oil, brown sugar, salt, and calamansi juice. Stir well until the mixture is smooth and well-blended.
At the 30-minute mark of baking, begin basting the chicken with the prepared sauce. Repeat the process every 5 minutes during the final 15 minutes of cooking, applying a generous layer each time for a flavorful, glossy finish.
For a charred and smoky touch, switch your oven to broil on high once the chicken is nearly done. Let the skin blister and slightly blacken, keeping a close watch to prevent burning.
Serve the Chicken Inasal hot, ideally over steamed rice, and enjoy the bold, zesty flavors of this Filipino classic.