In a medium bowl, sift together flour and salt. In a separate container mix egg and water. Place flour/salt and egg/water mixtures in the refrigerator and keep cold until ready to deep fry. It is important to keep batter and batter ingredients cold at all times so that the tempura stays crispy.
Begin heating the oil in your deep fryer of choice to 350 degrees.
Devein shrimp and remove all the shell except for the tail portion. Make several shallow slits on the underside of the shrimp until you are able to pull the shrimp flat. This will prevent the shrimp from curling up while cooking.
Cut the tip of the tail at and angle, then using the flat edge of the knife, press down on the tail to squeeze any excess water out the end of the tail. This will help prevent spattering during deep frying. Take the middle part of the tail shell off.
Dust the shrimp with cornstarch to absorb the smell and dirty water from the shrimp, then rinse the shrimp under running water.
Add sake to shrimp and rub them all over. Let sit for 10 minutes and then drain. There is no need to wash. Coat the shrimp in cornstarch.
Just before you are ready to deep fry, remove batter ingredients from the refrigerator. Add egg/water to flour mixture. Stir just to combine, some lumps will remain. Do not over mix and keep cold at all times. If possible, put bowl over an ice bath while you work. For crispy tempura, the goal is the prevent the activation of the wheat gluten. So work quickly and keep everything cold.
Coat the shrimp in batter and add to the deep fryer. Pay attention to the oil temperature and keep it steady at all times. As you add shrimp, the temperature will drop slightly, so only add a few at a time to allow the temperature to recover. Additionally, do not crowd the fryer. Cook the shrimp for 2 minutes and then remove to a wire rack or a plate lined with a paper towel to drain the excess oil. Between batches, remove any crumbs from the oil so that they do not burn and affect the taste of the oil.
Serve with unagi sauce for dipping.
If you are frying large batches, some of the first shrimp may loose their crispness over time. If this happens, place shrimp on a baking rack over a baking sheet and broil for a few minutes until batter recrisps.
To reheat leftovers, either put under the broiler as described above, or put in an Air Fryer and reheat at 375 degrees for 4 to 5 minutes until recrisped and heated through. Flip shrimp halfway through cooking time.