Wash the carambolas and cut them into slices. Clean and wash spring onions and cut them into large pieces. Peel garlic and ginger and chop finely. Clean chillies, cut lengthwise, remove seeds, wash and cut into fine rings.
Bring just under 1/2 l salted water to the boil. Cover the rice and cook on low heat for about 15 minutes.
Cut the prawns in the back and remove the dark intestine. Then wash and pat dry.
Heat butter and sugar in a large pan until the sugar caramelises to a golden yellow colour. Add the carambolas and spring onions and turn briefly. Add star anise, chilli, garlic and ginger.
Deglaze with soy sauce and about 150 ml water. Bring to the boil, cover and simmer at low heat for about 5 minutes. Stir from time to time.
Heat the oil in a large frying pan. Fry the prawns in it for about 1 minute on each side. Season with salt and pepper. Stir starch and 1 tablespoon cold water until smooth. Stir into the sauce, bring to the boil again and season to taste.
Leave the prawns to simmer briefly in the sauce. Arrange everything and garnish with Thai basil.