Mix the egg and potato flour in a bowl to form a batter. Heat a wok or pan over a high heat and fill to a quarter of its depth with groundnut oil. Heat the oil to 180°C (350°F), or until a cube of bread turns brown in 15 seconds. Dip the prawns in the batter one by one and lower them into the oil. Cook until golden, then remove from the oil and drain on absorbent kitchen paper. Place all the ingredients for the herby sauce in a container suitable for use with a hand stick blender and blend well. Drain the wok of oil, wipe clean with absorbent kitchen paper and return the wok to the heat. Add the sauce and bring to the bubble, then take off the heat. To make the crispy tea leaves, heat the groundnut oil in a small pan over a high heat, add the tea leaves and lightly fry until crisp. Remove and drain on kitchen paper. Place prawns on a serving plate, pour the hot sauce over them, sprinkle with the crispy tea leaves and serve immediately with steamed rice. Alternatively, serve the sauce as a dip for the prawns. Chinese Food Made Easy by Ching-he Huang, £16.99, HarperCollins