Slice each chicken breast crosswise, with the knife at an angle to the board, into 8 slices. Mix with the oil, garlic, spring onion, fish sauce, vinegar, chilli powder and sugar.
Heat a wok or large frypan and cook over high heat, stirring all the time, for about two minutes, until the chicken is milky white.
Mix together water, stock powder and cornflour. Remove the cooked chicken from the pan and keep it warm. Add the cornflour mixture to the pan and stir until sauce thickens and has boiled down to about half its volume. Return the chicken to the pan and toss to coat in the sauce.
Serve like this - or add Thai chilli sauce and stir to coat, then toss with the chopped coriander.
Serve immediately on rice or noodles, with a salad.