For the tempura prawns, place the vegetable oil into a deep, heavy-based saucepan and heat until a small cube of bread sizzles and turns golden when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Place the flour and beer into a bowl and whisk well until the batter is the consistency of single cream, adding more or less beer as necessary. Add the ice cubes and stir well.
Dip the prawns in the batter and then carefully place into the hot oil. Deep fry until golden-brown and cooked through. Remove with a slotted spoon and drain on kitchen paper.
For the mango salsa, place all the salsa ingredients into a bowl and mix well.
Serve the prawns on a plate with a bowl of the mango salsa to the side. Garnish with fresh coriander and lime wedges.