Combine lemongrass, ginger, onion, galangal and both types of chillies in a food processor. Process into a paste.
Heat oil in a wok, add the combined chilli paste and fry until fragrant. Add tamarind skin, sugar, peanuts and salt to taste. Stir over low heat until the gravy is thick and fragrant. Dish out to cool.
If you prefer a less sweet dish, reduce the sugar to suit your taste.