Cut the chicken thighs into bite sized pieces, trimming off any excess fat. Add chicken to a large bowl and add garlic powder, shaoxing wine, egg, salt, and white pepper. Mix together until everything is combined and chicken is fully coated. Set aside to marinate for at least 20 minutes.
While the chicken is marinating, make the sweet and sour cream. Add ketchup, pineapple juice, vinegar, chicken broth, soy sauce, and brown sugar to a saucepan over medium heat. Bring to a boil, whisking occasionally. Boil for 10-15 minutes, until the sauce reduces by about 40%. Whisk together cornstarch and water into a small bowl to create a slurry. Reduce the heat to low heat and add the slurry to the sauce and stir together until the sauce thickens and has a syrup-like consistency (about 3-5 minutes). Add butter and whisk until butter is melted, then set aside to cool.
In a large skillet over medium heat, add a drizzle of oil and saute bell peppers and onion for about 5-7 minutes, until slightly softened. Remove from the skillet and set aside.
To the same skillet, add three inches of vegetable oil and preheat the oil to 350-375 degrees f.
In a separate bowl - whisk together the dry ingredients for the batter - flour, baking powder, cornstarch, and salt. Then pour in the warm water and oil. Whisk until a smooth batter forms, about 10-15 seconds. Do not overmix, whisk until just smooth.
Pour the batter into the bowl with the marinated chicken and mix everything together. (the mixture may look soupy - this is OK).
Add chicken to the hot oil, fry in batches so as to not overcrowd the pan, and fry for 5 minutes, until the chicken begins to float to the top of the oil and no longer looks soggy. Remove and place onto a paper towel lined plate or wire rack.
Once all of the chicken is fried, add the chicken back to the oil again and fry for 3-4 minutes, until golden brown and crunchy.
Assemble everything - combine fried chicken, bell peppers and onions, and add the sauce. Toss together and sprinkle with sesame seeds for garnish.
Notes
Some recipes include canned pineapple chunks - not my thing, but if you like it, feel free to toss it in the sour sauce. Just be sure to drain off the juice first.
Cut Chicken Evenly into 1½-2 inch pieces so everything cooks at the same rate.
Pat the Chicken Dry by blotting the chicken with paper towels. Dry chicken holds into the seasonings and breading coating better. This gives you extra crispy chicken!
Use a candy thermometer to ensure that the oil temperature is correct. You want to maintain 350-375 degrees F. This prevents soggy chicken and gives you that nice crispy exterior while maintaining juicy chicken on the inside.
If you let the sauce simmer too long and it becomes thick, add a splash of water to thin it out.