Measure out your water and cake flour and store in the fridge while you prepare the shrimp. We want all the batter ingredients to be cold while we work with them.
Begin preparing the shrimp by peeling and deveining the shrimp. Leave the segment of the shell right before the tail. To devein the shrimp, you can pierce part of the back toward the tail with a toothpick and carefully pull out the vein. Or you can cut a slit down the back and remove it like that as well. Do whatever is easiest or works best for you.
Next, rip off the pointy part of the tail and lay the shrimp down so that the tail folds in half symmetrically. Cut the tip of the tail diagonally and use the back of the knife to remove the water and impurities in the tail.
Make 4-5 evenly spaced shallow cuts on the underside of the shrimp on each segment of muscle. Then flip the shrimp over so the cuts are facing the cutting board and firmly press with your fingers until you hear a snapping sound. This is enough to straighten shrimp, but you can also gently pound with the side of your knife for good measure.
Next we will clean the shrimp. Lightly coat the shrimp with starch and rub around. Then rinse with water until the water is clear. Dry off with a paper towel.
At this point you can coat the shrimp and any other ingredients you would like to make tempura of with flour and heat the oil to 185°C/370°F.
Now it is time to make the matter. You want to work quickly to make sure everything stays cold. Crack the egg into the water and whisk until dissolved. Add the liquids to the cake flour and roughly mix with chopsticks to prevent overmixing. You want some lumps left in the batter.
Once the oil is hot, dip the shrimp into the batter and lay into the frying oil. At this point you can also drizzle some batter over the shrimp and push/roll the shrimp into it. This makes the shrimp prettier and adds more crispy batter bits around, but it’s not necessary! Use a spider to gather and remove any excess floating fried batter bits to keep the oil clean. These bits are called tenkasu and can also be used to top your noodles!
Once the shrimp has finished frying, remove it from the oil and let it cool on a cooling rack. It’s best if they cool standing upright, but it’s okay if you have to lay it flat. Continue frying until all the shrimp are cooked and let cool.