To make the dipping sauce, peel the papaya, remove the seeds and chop it roughly. Put it in a blender with the other ingredients and process until smooth. Pour the sauce into a pan and bring it to the boil, then immediately turn down to a simmer and cook gently for about 5 minutes. If the sauce gets too thick, add a few tablespoons of just-boiled water. For the batter, sift the plain flour, baking powder and salt into a bowl. Make a well in centre and break in the egg, then bring in the flour from the sides. Whisk in the cold water to make a smooth batter. Put the coating flour in a shallow bowl. Put the batter in a second bowl and the breadcrumbs mixed with coconut in a third. Season the prawns with salt and pepper, then coat a prawn in flour, shaking off any excess and holding it by the tail, dip it into the batter. Lift it out and let any excess batter drip back into the bowl. Roll the prawn in the crumbs and coconut mix, pressing it down so the mixture sticks. Put the prawn on a plate lined with greaseproof paper and repeat until all are coated. Leave some space between each one. Add the oil to a large pan and heat to 180°C. Deep-fry the prawns – a few at a time – for 1 or 2 minutes until golden and crisp. Drain them on kitchen paper. Serve at once with the papaya dipping sauce.