In a shallow dish, mix together the sugar and fish sauce to make the marinade. Toss in the chicken, turning to coat. Let sit for 15 minutes.
Mix the cornflour slurry and oil in with the chicken.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir-fry the chicken until done, about 4-5 minutes. Set aside.
In the same frying pan, bring the coconut milk, lemon grass and Thai basil to the boil over high heat. Once boiled, turn down the heat to medium, cover and simmer for 10 minutes. Pass through a sieve and set aside.
Wipe the pan clean. Heat the pan with 1 tablespoon oil over high heat. Stir-fry the green curry paste until fragrant, 2-3 minutes. Stir in the onion and salt and cook until soft, 3-4 minutes.
Add the chicken, aubergine and coconut mixture and stir well. Bring the mixture to the boil, then turn the heat down to medium. Cover and simmer for 4 minutes. Turn off the heat and stir in the basil. Serve.
Thai aubergine and Thai basil leaves are available in Chinese/Oriental speciality stores or online.