Heat a heavy frying pan, add the chili powder, fennel seeds, peppercorns, start anise and cinnamon sticks and dry-fry for 1-2 minutes to release the flavours.
Leave to cool, then grind coarsely in a grinder or tip into a mortar and crush with pestle.
Heat the peanut oil in a shallow pan, add the garlic, ginger and chopped shallot and cook over a low heat, stirring occasionally, until very lightly colored.
Add crushed spices, season with salt and pepper and the cook the mixture gently for 5 minutes.
Add rice vinegar and brown sugar, stir until dissolved, and then add the prawns.
Cook for about 3-5 minutes, until they turn pink, but are still juicy.
Serve hot and garnish with lime wedges and green onion.