In a small bowl, sift the flour once or twice to remove any clumps and to make it light and soft. Set aside.
In a separate medium bowl, gently beat egg until the yolk and egg whites are just barely incorporated.
Combine the water and ice cubes in a cup. Using a strainer, strain the water (you should have 1 cup of ice-cold water) and add it to the bowl with the beaten egg. Make sure you do not actually add ice cubes to the tempura batter.
Add the sifted flour into the bowl with the egg and water mixture and lightly combine the flour using chopsticks. Be careful not to overmix the batter; it should be a little lumpy.
Notes
How to Use
When ready to use your tempura batter, there are a few things to keep in mind:
First, lightly coat the seafood or vegetable in either cake flour, Wondra flour, or all-purpose flour before dipping them into the tempura batter. This allows the batter to adhere better.
Once coated, dip your items into the batter gently. Too much batter runs the risk of a crispy exterior and mushy interior.
When ready to fry, make sure that your frying oil is between 340 F and 360 F. Any higher and it will be too crispy. Any lower and the tempura will absorb too much oil and won't get crispy enough.
Once fried, serve immediately with a dipping sauce and dig in. Tempura can get mushy if it sits too long.
If for some reason the batter won't be used right away, place it in the refrigerator temporarily (for a few short minutes) to keep it ice cold until you're ready to deep-fry your tempura. Do not store in the fridge for an extended period of time.
Tips
Prepare all of the ingredients you plan to deep-fry prior to mixing the batter ingredients.
Always sift the flour. This makes the flour lighter and easier to incorporate into the batter when it's mixed.
For crisp tempura, use ice water instead of room-temperature or tap water.
Instead of a whisk, use chopsticks to mix the tempura batter ingredients. This minimizes the amount of air in the batter and lessens the risk of overmixing.
Heat the oil for deep-frying before the tempura batter is prepared to ensure the batter is at its coldest when it hits the oil and that the oil is ready for frying.
While frying, do not place the bowl of batter on the hot stove or it will get too hot.
Don't prepare the tempura batter ahead of time, as it will not yield the best results.