- Combine flour, 1½ tsp salt, turmeric, cayenne, spring onions, egg, soda water and iced water in a bowl and mix until just blended. Do not overmix. The batter will be slightly lumpy. Add 1 tbsp iced water at a time if the batter is too thick.
- Place prawns in a separate bowl and season with a pinch of salt and black pepper. Add 120ml batter over the prawns and mix well to coat evenly. Then, add sweet potatoes to the bowl of remaining batter and mix well.
- Heat enough oil for deep frying in a wok or deep-frying pan over high heat. Place round metal rings into the oil and pour in the sweet potato batter into each metal ring and top with a battered prawn each. Fry until golden browned and cooked through. Drain off excess oil. Repeat with remaining batter.
- Serve with lettuce leaves and sweet chilli sauce.