Place the chopped up chicken thighs in a medium sized bowl, and add 1 ½ tablespoons of soy sauce, 1 ½ tablespoons of sesame oil, 1 teaspoon of brown sugar, ½ teaspoon of powdered garlic, and ½ teaspoon of black pepper. Mix everything together and then set the chicken aside to marinade for 10 minutes.
Preheat a deep fryer to 350 F, or heat up oil in a large pan; the oil should be at least 1-inch deep. Use a cooking thermometer to keep track of the oil's temperature.
Meanwhile, in a small saucepan, add the pineapple juice, the rice wine vinegar, the ketchup, 2 tablespoons of soy sauce, ½ cup of brown sugar, ¼ teaspoon of garlic powder, and a pinch of chili flakes.
Heat up the saucepan to a medium heat until the ingredients are simmering, stirring regularly. When all of the sugar has dissolved and the ingredients are combined, mix together 1 tablespoon of cornstarch in a small bowl with a little water to form a slurry, then add it to the saucepan. Stir the slurry into the sweet and sour sauce, and allow it to simmer for a few minutes longer as it thickens. Once the sauce thinly coats the back of a spoon, turn the heat off and set the sauce to the side.
When the fryer or the oil is up to temperature, add the egg white to the bowl with the marinading chicken and mix it in. Then, add 1 cup of cornstarch and thoroughly mix that into the chicken until it is coated.
Place the cornstarch-coated chicken into the deep fat fryer or oil, and allow it to fry for a few minutes until it is golden all over and cooked through. Once cooked, remove the chicken and place it into a kitchen towel-lined bowl, to absorb any excess oil.
In a large frying pan or wok, heat up 1 tablespoon of frying oil to a medium-hot temperature. Add the chopped onion, red pepper, and green pepper to the pan and fry for 3 minutes. Then add the matchsticks of ginger and ¼ teaspoon of garlic powder, and fry everything for a minute longer until fragrant. Add the fried chicken to the pan, along with the sweet and sour sauce, and the pineapple chunks. Mix everything together and cook until it is heated through, about 3 to 4 minutes. Serve hot and fresh from the pan, with rice and greens, and a sprinkling of toasted sesame seeds.