Use 1/2 cup each of flour and cornstarch beaten together with an egg and 1 cup of seltzer water to make an ideal tempura batter. While traditional Japanese tempura is a simple mixture of flour, eggs and ice water, home cooks can achieve the crispest texture by adding cornstarch as well and by using seltzer water in place of plain water.
For best results, wait to make the batter until the rest of the ingredients are all prepared. The oil should be at the correct temperature for frying, which is normally between 360 and 375 degrees Fahrenheit, and the vegetables or seafood should be cut into bite-size pieces. Mixing the batter just before frying will prevent the liquid from absorbing into the starch and making the tempura doughy or gummy. Be sure to leave a few lumps in the mixed batter. Mixing the tempura batter too thoroughly can result in a finished product that is tough and chewy, rather than crisp and delicate.
Once the batter is prepared, dip the vegetables or seafood into it, and fry it in the oil until the batter has formed a golden crust. Only add a few pieces to the pan at a time. Overcrowding will make the temperature of the oil drop too quickly, leading to a greasy texture.