Prepare all ingredients by cutting the chicken into bite-sized pieces and measuring out the sauce ingredients. Set aside.
Mix the pineapple juice, soy sauce, vinegar, and cornstarch in a small bowl until the cornstarch is fully dissolved. This will be your sauce base.
Heat the wok or skillet over medium-high heat and add a tablespoon of oil. When shimmering, add the chicken pieces and cook, stirring occasionally, until they develop a light golden exterior, about 5-6 minutes.
Pour the prepared sauce over the cooked chicken, stirring quickly to coat all pieces evenly. Let the sauce simmer and thicken, about 3-4 minutes, until it becomes glossy and sticky.
Add the pineapple chunks to the wok, stirring to distribute them among the chicken. Cook for another minute until heated through and fragrant.
Sprinkle the toasted sesame seeds over the top of the dish just before serving to add a nutty aroma and crunch.
Serve the pineapple chicken hot, garnished with additional sesame seeds if desired, and enjoy the vibrant flavors and textures.