In a mixing bowl, whisk together soy sauce, brown sugar, garlic, ginger, and pineapple juice until sugar dissolves.
Place chicken in a zip-top bag or shallow dish; pour in half the marinade (reserve other half). Seal and refrigerate for at least 30 minutes, up to 4 hours.
Preheat grill or skillet over medium-high heat. Remove chicken from marinade and let excess drip off. Discard used marinade.
Grill or pan-sear chicken for 6–7 minutes per side, or until cooked through and browned.
Meanwhile, pour reserved marinade into a small saucepan; bring to a simmer.
Mix cornstarch and water to form a slurry. Add slurry to simmering marinade; cook, stirring, until thickened and glossy—about 2 minutes.
Brush cooked chicken with thickened teriyaki sauce. Top with green onions and optional pineapple chunks. Serve hot and enjoy!
