Place all the marinade ingredients into a bowl and add the chicken. Coat the chicken pieces thoroughly and leave to marinade for 20 minutes.
Chop up the garlic and ginger finely. You could also grate the ginger if you prefer.
Slice the spring onions (scallions) in small rings separating the white and green in two bowls.
Place the dry coat ingredients onto a flat plate and mix thoroughly.
Add all the sauce ingredients in a bowl, whisk with a fork to mix, and set aside.
Heat the 250ml (1 cup) of oil in a wok or deep frying pan on high heat until it's hot and just smoking. Lower the heat to medium high (that means about 3/4 of the way).
Take the marinated chicken pieces, and dip them into the dry coat one at a time. Shake off excess flour and carefully lower it into the hot oil. Don't drop it in, you might get splashed with the hot oil. Use a spatula to lower the chicken in, if it worries you. Don't overcrowd, fry 5-6 pieces at a time.
Cook the chicken for 3 minutes, turning, if not completely covered by the oil. When done, drain on a plate lined with kitchen paper.
When all the chicken pieces are cooked, pour out all the oil but 1 tablespoon of it and place back on the stove and lower the heat to medium.
Add the chillies, garlic, ginger and white spring onions (scallions) and fry for 30 seconds.
Stir the sauce that you had set aside earlier with a spoon, as the cornflour would have settled to the bottom. Now add this sauce to the wok and bring to a boil, stirring as it thickens (because of the cornflour).
Add the chicken pieces and stir for 30 seconds, still on medium heat, and coat the chicken pieces thoroughly.
Take off the heat and stir in the sesame oil.
Dish out onto a warm plate and scatter the green spring onion (scallions) slices and sesame seeds all over and serve immediately.