For the tempura prawns, place the vegetable oil into a deep, heavy-based pan and heat until a small cube of bread dropped into it sizzles and turns golden-brown (CAUTION: Hot oil can be dangerous. Do not leave unattended).
Mix the flour and water together in a bowl to make a thin, smooth batter. Dip the prawns into the batter to coat.
Carefully drop the prawns into the hot oil and deep fry for 3-4 minutes, or until golden-brown and crisp. Remove from the oil with a slotted spoon and drain on kitchen paper.
For the mango, chilli and lettuce cups, mix the mango, lime zest and juice and chilli together in a bowl.
Place the lettuce leaves onto a plate. Spoon the mango mixture into the middle of each leaf and top with a prawn. Serve immediately.