Peel the prawns, leaving the tails on, and take out the black vein that runs along the back of each prawn. Leave the prawns in a cool place, covered, while you make the batter and sauce.
To make the batter, put the flour, salt and olive oil into a bowl and make a well in the center. Whisk the egg yolk with the coconut milk, and gradually add to the flour, beating well. Cover and set aside.
To make the mango sauce, peel the mangoes and remove the stones. Cut up the flesh and process in a food processor or blender, then turn into a saucepan, add the water and stir over a low heat until blended. Cover and simmer for 15—20 minutes until thickened. Stir in the sugar, spices, lime juice and salt and leave to cool.
Whisk the egg white until stiff and fold into the batter. Add the prawns. Heat the oil for deep-frying to 190°C 375ºF) and fry the prawns, in batches, until golden brown. Remove and drain on absorbent kitchen paper. Serve immediately with the sauce.