Peel and devein the shrimp and rinse under cold water. Pat the shrimp dry with paper towels.
Make 4-6 shallow slits on the underside of the shrimp.
Place the shrimp right side up and push against the back of the shrimp with your index finger to break the muscles. Use your thumb and middle finger to hold it in place as you press. You should hear popping sounds as the muscles break and the shrimp straightens out. Repeat with the remaining shrimp.
Make the Tempura Batter
In a small bowl, whisk together the egg and ice cold water until it's well incorporated.
In a separate mixing bowl, whisk together flour and cornstarch. Pour in the egg/water mixture.
Use a whisk or chopsticks to gently mix until the dry ingredients are incorporated with the wet. Do not overmix. It's okay for the batter to be lumpy.
Keep the batter in an ice bath until you're ready to fry.
Fry Ebi Tempura
Heat 1.5"-2" of oil, in a pot or Dutch oven, over medium heat until it reaches 375°F. I like to use cast iron when frying since it helps maintain the temperature of the oil as you add ingredients.
Lightly dredge the shrimp with flour and shake off the excess.
One at a time, dip the shrimp into the batter and allow the excess to drip off. Carefully lower the shrimp into the oil. Cook the shrimp in batches to avoid overcrowding, no more than 3-4 at a time, depending on the size of your pot.
Slowly drizzle 2-3 spoonfuls of batter in a zigzag pattern across the length of each shrimp. Some batter will stick and some will bubble and float away. Carefully move the shrimp around the oil to catch the batter.
Fry the shrimp tempura until it's golden brown (about 2 minutes).
Remove the shrimp from the oil and place on a wire rack or paper towels to allow excess oil to drip off.
Repeat with the rest of the shrimp. In between batches, use a strainer to remove the excess bits of batter that did not stick. You want to have clean oil when starting a new batch.
Notes
Before frying, ensure the shrimp are thoroughly dry.
Keep the batter in an ice bath until you're ready to start frying.
Be sure to use a strainer to clean out all the little bits of batter from the oil in between batches.