Add enough vegetable oil to a deep-sided skillet or large pot so that it comes 1 inch up the sides. Place over medium to medium-high heat and heat the oil until it reaches 350 degrees F.
In a shallow bowl, whisk together the flour, cornstarch, baking powder, and salt. Then whisk in the egg and water until the batter is smooth. Set aside.
Use a sharp knife to cut horizontal slits into the underside of each shrimp. This will help the shrimp straighten out while cooking (instead of being curved).
Dip the shrimp into the tempura batter. Then, immediately add it to the hot oil. Cook for 2-3 minutes, flipping once, until golden brown. (I can usually get 2-3 shrimp in the pan, you don't want to overcrowd it.)
Use tongs to remove the shrimp to a paper towel-lined plate to drain. Repeat with the remaining shrimp.
Serve warm with tempura dipping sauce or yum yum sauce.
Notes
I like 16/20 (extra jumbo size) shrimp best for this recipe. If you use a smaller size like jumbo or large, you get a higher recipe yield. There should be plenty of batter for this adjustment if needed.