As with all shellfish, discard any pipi that are already open in their shells before cooking - they've gone bad.
- Immerse pipi in fresh water and leave overnight. Drain off water and make sure sand is rinsed off. Place pipi in a saucepan with ¼ cup water. Cover and place over a medium-high heat for 3 to 4 minutes, shaking the pan at times, until pipi have opened.
- Remove meat from shells and roughly chop. Place flour in a bowl and add chopped onion, lemon rind, chopped parsley and season with salt and pepper. Mix in the sweet chili sauce, soda water and pipi meat.
- Heat butter and oil together in a large frying pan and cook heaped teaspoonfuls of batter over a medium heat until golden on both sides.