Wash and pat dry the chicken before cutting it into bite-sized pieces.
Season with salt and black pepper.
Mix cornflour, flour, baking powder, egg, and chilled water to make a smooth batter.
Coat each chicken piece evenly in the batter.
Heat vegetable oil in a wok or deep frying pan.
Fry the chicken in small batches until golden and crispy.
Remove and drain on kitchen paper.
Stir-fry the onion, peppers, garlic, and ginger for 3–4 minutes.
Mix ketchup, sugar, vinegar, soy sauce, pineapple juice, and cornflour slurry in a bowl.
Pour the sauce into the vegetables and simmer until glossy.
Add pineapple chunks.
Return the fried chicken to the wok and toss until fully coated.
Cook for another 1–2 minutes.
Garnish with spring onions and sesame seeds.
Serve immediately with steamed rice or noodles.