In large bowl, toss chicken with ½ tsp of the salt and the pepper. Cover and refrigerate for at least 2 hours or for up to 8 hours.
In separate large bowl, whisk together cornstarch, tapioca starch, custard powder, sugar and 1½ tsp of the salt. Stir in egg, cooking wine and ⅓ cup of water. Add chicken, turning to coat.
In deep-fryer or Dutch oven, add enough oil to come 3 inches up side over medium-high heat until deep-fryer thermometer reads 375°F.
Using slotted spoon, carefully lower chicken pieces, 3 to 4 at a time, into oil. Fry chicken, turning occasionally, until juices run clear when thickest part is pierced and instant-read thermometer inserted in thickest part of a few pieces reads 180°F, about 10 minutes. Drain on rack set over paper-towel-lined baking sheet. Repeat with remaining pieces.
In large bowl, toss fried chicken with remaining ½ tsp salt and white pepper.
Arrange on platter and sprinkle with chilies, cilantro and green onion. Serve with lemon wedges and sweet chili sauce (if using).
Notes
Tip: Custard powder in fried chicken? You bet! It’s a delightful traditional way to add a hint of sweetness and help create a delectable crunchy exterior.