Season chicken: pat dry. Season both sides with salt, pepper, and garlic powder.
Sear: heat oil in a large skillet over medium-high. Sear chicken 3 to 4 minutes per side until golden. Remove and set aside.
Aromatics: reduce heat to medium. Add garlic and ginger. Cook 30 seconds until fragrant.
Make sauce: add reserved pineapple juice, brown sugar, soy sauce, and vinegar. Stir and bring to a simmer.
Thicken: whisk cornstarch with cold water. Pour into sauce while stirring. Cook 2 minutes until thickened.
Finish: add pineapple chunks and return chicken. Spoon sauce over chicken. Cook 6 to 8 minutes until chicken reaches 165 degrees F.
Serve: garnish with green onions and sesame seeds. Serve over rice or noodles.