Prepare Annatto Oil: In a small saucepan, heat 0.5 cup vegetable oil over low heat. Add annatto seeds and cook for 5-7 minutes, stirring occasionally, until the oil turns a deep orange-red. Strain the oil into a heatproof bowl, discarding the seeds. Set aside to cool.

Prepare Chicken Marinade: In a large bowl or resealable bag, combine the chicken leg quarters, cane vinegar, 0.25 cup calamansi juice, minced garlic, grated ginger, pounded lemongrass, black pepper, brown sugar, and 1 teaspoon salt. Mix well to ensure the chicken is thoroughly coated. Cover and refrigerate for at least 4 hours, or preferably overnight, to allow flavors to meld.

Prepare Basting Sauce: In a separate bowl, mix 0.25 cup of the prepared annatto oil with the melted butter, 2 tablespoons calamansi juice, and 0.5 teaspoon salt. Stir until well combined. Set aside.

Grill the Chicken: Preheat a charcoal grill to a medium-high heat. Lightly oil the grates to prevent sticking. Remove the chicken from the marinade, shaking off any excess aromatics. Do not wipe dry.

Grill the chicken for 30-40 minutes, turning every 5-7 minutes, until cooked through and tender. During the last 15-20 minutes of grilling, frequently baste the chicken generously with the annatto-butter sauce using a brush, ensuring an even coating and beautiful color. Continue to turn and baste until the chicken skin is golden-brown and slightly charred in places, and the internal temperature reaches 165°F (74°C).

Serve hot with steamed white rice and a dipping sauce made from sinamak (spiced vinegar) if desired.