Place the chicken and cornflour into a bag or bowl and toss to coat. Heat oil in a wok or large frypan over medium-high heat. Fry the chicken pieces and garlic, stirring constantly until lightly browned. Stir in the soy sauce, vinegar, sugar and water. Cover and cook until the chicken pieces are no longer pink inside, 3 to 5 minutes.
Stir in the spring onion and cayenne pepper, cook uncovered for about 2 more minutes. Serve over white rice.