To make the dipping sauce, combine all ingredients except peanut oil in a heatproof bowl. Heat peanut oil in a small heavy-based pan until surface seems to shimmer slightly, then carefully pour into bowl to release flavours.
Stir to combine and set aside.
Prawns
Bring a large pan or stockpot of water to the boil. Add prawns and simmer for 2 minutes, then drain immediately.
Serve prawns while still warm, with a small bowl of dipping sauce alongside.