Season the curry with fish sauce and brown sugar, then pour in the rest of the coconut milk. Mix everything thoroughly and add half of the makrut lime leaves. Bring it to a simmer and cook for 3-5 minutes, or until the chicken is cooked through.
Taste the curry and adjust the seasoning with more sugar or fish sauce if needed; you're looking for a balance of salty, nutty, and a hint of sweetness. Finally, stir in the Thai basil leaves, give it a quick mix, and take it off the heat. Serve immediately with steamed jasmine rice, garnished with sliced fresh chilli and the remaining makrut lime leaves.