In a bowl, whisk together the pineapple juice, brown sugar, BBQ sauce, chili powder, red pepper flakes, soy sauce, and garlic powder until the sugar is fully dissolved and the spices are evenly distributed. This creates a balanced sweet-spicy-savory marinade that will coat the chicken and caramelize beautifully on the grill.

Place each chicken breast between plastic wrap and pound gently to an even 1/2-inch thickness—this ensures uniform cooking and faster marinating. Transfer the pounded chicken to a large resealable bag or shallow dish, pour in the marinade from Step 1, seal, and refrigerate for 2 to 4 hours, turning the bag halfway through if possible. I like to set a timer at the 2-hour mark so the chicken gets maximum flavor without over-marinating.

Preheat your grill to medium-high heat (about 400°F). Remove chicken from the marinade, letting excess drip off, and arrange on the grill grates. Grill for 12 to 15 minutes, turning once halfway through, until the internal temperature reaches 165°F and the exterior is caramelized with light charring from the brown sugar. The marinade's sugars will create a beautiful glaze as the chicken cooks—don't move the chicken too often so this crust can properly develop.

Transfer the grilled chicken to a plate and let it rest for 5 minutes before serving. This allows the juices to redistribute, keeping the meat tender and juicy.