Season the chicken with garlic salt, and place the cubed chicken into a bowl or zip lock bag. Add the cornstarch, and toss or shake to coat the chicken well.
Heat the oil in a large pan over medium high heat. Cook the chicken on both sides for 6-8 minutes. Stir occasionally until chicken is browned and mostly cooked through. It will continue to cook for additional time while it simmers in the sauce.
Add the brown sugar, pineapple juice, soy sauce, vinegar and cornstarch to a medium sized bowl, and whisk until incorporated. Make sure the cornstarch is dissolved.
Pour over the browned chicken in the pan.
Add the pineapple chunks, and toss the sauce to coat the chicken in the pan.
Continue to simmer and cook on low, and occasionally stir the sauce and chicken. Cook until the sauce thickens and the chicken is cooked through. This should take about 15 minutes.
Serve over rice, and garnish with sesame seeds and diced green onion.