1. Wash and drain the chicken pieces/fillets. Pat them dry if necessary and cut them into bite-sized pieces. You can use a mix of boneless dark and white meat for moist and juicy chicken pieces.
2. Marinate the chicken with salt and pepper powder and set it aside for 20 minutes.
3. In a small bowl, combine all the ingredients for the sweet and sour sauce (tomato ketchup, sugar, white vinegar, and dark soy sauce). Mix well and set it aside.
4. Prepare the batter by combining all the batter ingredients (maida/all-purpose flour, cornflour, baking powder, salt, egg white, and chilled water) in a bowl. Mix until you get a smooth batter.
5. Heat oil for deep frying in a skillet, wok, or kadai.
6. Dip the marinated chicken pieces in the batter and carefully drop them into the hot oil, making sure not to overcrowd the pan. Fry the chicken on medium heat for 1 minute without turning them, then turn and fry for another 4-5 minutes until they turn golden brown. Remove the fried chicken from the oil and set it aside. Repeat the process with the remaining chicken pieces.
7. In a wok or kadai, heat 1/2 tablespoon of oil over high heat. Add the cubed green and red bell peppers and stir-fry for 1 minute.
8. Add the sweet and sour sauce mixture to the wok and continue to stir-fry on high heat for another minute until the sauce is cooked and thickened.
9. Add the fried chicken pieces to the wok and mix well, coating the chicken pieces evenly with the sauce. Stir-fry for about 2 minutes to allow the flavors to blend.
10. Serve the sweet and sour chicken with noodles or fried rice.