Step 1: Prepare the Marinade

1. In a medium mixing bowl, whisk together soy sauce, pineapple juice, brown sugar, ketchup, vinegar, garlic, ginger, and chili flakes (if using). Taste and adjust sweetness or acidity to your preference.

Step 2: Marinate the Chicken

1. Place the chicken pieces in a large zip-top bag or shallow dish. Pour half of the marinade over the chicken, reserving the other half for basting and as a finishing sauce.
2. Seal the bag or cover the dish, and refrigerate for at least 1 hour, or up to 8 hours for maximum flavor.

Step 3: Preheat and Prepare for Cooking

1. If grilling, preheat the grill to medium-high heat and lightly oil the grates.
2. If baking, preheat the oven to 375°F (190°C) and line a baking sheet with foil for easy cleanup.
3. If pan-frying, heat olive oil in a large skillet over medium heat.

Step 4: Cook the Chicken

Grilling Option

1. Remove the chicken from the marinade and place it on the preheated grill. Discard the used marinade.
2. Grill the chicken for 6-8 minutes per side, or until the internal temperature reaches 165°F (74°C). Baste occasionally with the reserved marinade during the last few minutes of cooking.

Baking Option

1. Arrange the chicken on the prepared baking sheet and bake for 20-25 minutes, flipping halfway through. Brush with reserved marinade during the last 10 minutes for added flavor.

Pan-Frying Option

1. Heat olive oil in a skillet over medium heat. Add the chicken and cook for 5-7 minutes per side, or until golden brown and fully cooked. Spoon the reserved marinade over the chicken in the last few minutes to create a glaze.

Step 5: Garnish and Serve

1. Transfer the cooked chicken to a serving platter. Garnish with fresh pineapple chunks, sliced green onions, and toasted sesame seeds if desired.
2. Serve warm with your choice of sides.