To prepare the marinade, process the lemongrass, garlic, ginger, and salt together in a food processor until fine in texture.
In a bowl, combine the processed aromatics mixture with the annatto oil, brown sugar, cane vinegar, calamansi juice, liquid seasoning, and ground black pepper. Mix well to combine.
Place the chicken thighs inside a plastic bag and pour the prepared marinade on top. Make sure the marinade coats all the chicken pieces adequately. Let marinate for at least 1 hour to a maximum of overnight.
When ready to cook in the oven, place the marinated chicken on a wire rack and bake in a preheated oven at 375°F for a total of 45-50 minutes, or until the internal temperature is 165°F.
Meanwhile, prepare the basting sauce by combining all the ingredients together until smooth. In the last 15 minutes of cooking, you'll need to baste the chicken every 5 minutes (for a total of 3-4 bastings).
Once the chicken is just about cooked, turn the broiler on high, and let the chicken broil until the its skin begins to char. Serve immediately with rice and fresh veggies, if desired.
