The crispiest foods depend on having the oil at the right temperature and frying the foods until crispy. For best results, I recommend a deep fry thermometer to monitor the temperature as you fry batches of battered green beans. The dipping sauce can be prepared up to 3 days in advance.
For the dipping sauce, blend the yogurt, chives, parsley, mint, zest, garlic salt, and cayenne, if using, in a bowl and taste for seasoning. Store in the refrigerator until ready to serve.
To make the batter, beat the egg in a mixing bowl and whisk in the rice flour and salt. It will become lumpy. When the oil is hot, add the sparkling water and whisk until smooth.
Grasp a bean by the stem, dip it into the batter and shake off excess. Drop it into the oil and repeat, adding just as many beans as can float freely in the oil.
Fry for about 3 minutes until crisp. (The rice flour will not brown. To test, pull a sample from the oil and test with your fingers to be sure it's crispy enough, bearing in mind that they will soften as they cool.) Use a wire skimmer or fork to fish out the beans onto the cooling rack and season with the sea salt.
Place the baking sheet in the oven to keep the beans warm while you repeat with the remaining beans. Serve with the reserved dipping sauce.