Prepare the chicken and batter. Pat the chicken pieces dry with kitchen paper. In a large bowl, whisk together the egg, cold water, plain flour, cornflour, salt, and white pepper until you have a smooth, thick batter that coats the back of a spoon. Add the chicken pieces and stir well, ensuring every piece is completely coated. The batter should look glossy and cling to the chicken without dripping off too quickly.

First fry for tenderness. Heat about 2cm of oil in a large frying pan or deep skillet over a medium-high heat. You'll know it's ready when a drop of batter sizzles immediately upon hitting the oil. Working in batches to avoid crowding, carefully lower the chicken pieces into the hot oil. Fry for 4-5 minutes, turning occasionally, until the batter is a pale, golden blonde colour and the chicken is cooked through. The coating will feel soft to the touch. Transfer to a wire rack set over a baking tray. Do not pile them up, or they'll steam and lose their nascent crunch.

Second fry for shattering crispness. Once all the chicken has had its first fry, increase the heat to high and let the oil get a little hotter - you should see faint wisps of smoke. Return all the chicken pieces to the pan in a single layer and fry for another 2-3 minutes, until the coating turns a deep, rich golden brown. You'll hear the sizzle intensify, and the pieces will feel hard and brittle when you tap them with your spatula. Remove and drain on fresh kitchen paper.

Make the sticky orange sauce. While the chicken rests, wipe out the frying pan and place it over a medium heat. Add the fresh orange juice, orange zest, brown sugar, soy sauce, rice vinegar, minced garlic, and grated ginger. Stir everything together and bring it to a gentle simmer. You should see small bubbles breaking across the surface, and the smell of citrus and ginger will fill your kitchen. Let it bubble for 2-3 minutes, until it reduces slightly and becomes syrupy.

Thicken the sauce. Give your cornflour slurry a quick stir, then pour it into the simmering sauce while whisking constantly. Within 30 seconds, you'll see the sauce transform from a thin liquid to a thick, glossy glaze that coats the back of a spoon. It should look like warm, clear honey. Taste and adjust the sweetness or tang with a pinch more sugar or a splash of vinegar.

Toss and serve. Add the crispy chicken pieces to the pan with the sauce. Using a wooden spoon or spatula, gently toss everything together until each piece is evenly coated in the glossy, amber-coloured sauce. Work quickly - this should take no more than 30 seconds to keep the chicken from softening. Transfer to a serving plate immediately, sprinkle with sesame seeds and sliced spring onions, and serve at once.