In a medium bowl, whisk together brown sugar, soy sauce, ketchup, apple cider vinegar, garlic powder, ginger, and pepper until well combined. Place the boneless skinless chicken thighs in a large sealable bag or shallow dish, then pour the marinade over the chicken, ensuring each piece is coated. Let it marinate for at least 30 minutes (up to 2 hours) in the refrigerator for enhanced flavor.

If you're using fresh pineapple, now is the time to chop it into bite-sized chunks. Alternatively, if you chose canned pineapple, drain the liquid thoroughly to prevent excessive moisture in your dish. Set the pineapple aside.

Heat a drizzle of oil in an oven-safe skillet over medium-high heat. Once the oil shimmers, carefully add the marinated chicken thighs to the skillet, allowing them to sear for 3-4 minutes on each side until golden brown and caramelized.

Pour the reserved marinade over the top, ensuring it covers the chicken evenly. Gently add the prepared pineapple chunks, distributing them around the chicken.

Preheat your oven to 375°F (190°C). Transfer the skillet with the chicken and pineapple mixture to the oven and bake for 25-30 minutes.

Once the chicken is done baking, remove it from the oven and let it rest for about 5 minutes.