Bring a small pot of salted water to a rapid boil over as high a heat you can get, and add just a small handful of green beans at a time so as to maintain the water boiling. Let for as little as 30 seconds or up to 2 minutes depending on the size of the beans. The beans should be cooked on the exterior with a crunch and firmness at the center. Shock the beans in a bowl of ice and water to stop the cooking. Repeat with the remaining beans. When beans are cold, drain all the ice water and pat the beans dry with a paper towel. Reserve.
Sift together into a bowl the first measure of flour, cornstarch, baking powder, and salt. In a separate bowl, whisk together the water, egg, and vegetable oil. Add the liquid to the dry ingredients, whisk until smooth. The batter should be a little thinner than pancake batter. If too thick, add more water, if too thin, add more flour. Reserve.
Heat a deep fryer with vegetable oil to 350ºf. Place the green beans in the batter, tossing to coat evenly. Let excess batter drip off and carefully place in the panko bread crumbs. Press gently to adhere the crumbs and add beans to the fryer. Fry only 8-10 beans at a time until a golden color. Drain on paper towels and season with salt and pepper if desired. Great served with thai sweet chile sauce, soy sauce, ponzu sauce or ranch dressing. Serves four.