Defrost shrimp overnight in your refrigerator. Take out when ready to cook.
In a clean deep pan, pour your choice of cooking oil to around 1.5-2 inches high, or just enough for deep frying. Heat the oil over medium heat until the temperature reaches 175 °C, for around 20 minutes. *Note: If without a thermometer, dip the end of a wooden spoon or chopstick into oil. If the oil steadily bubbles around the utensil, it should be ready. If bubbling is too vigorous, oil is too hot.
Meanwhile, transfer flour for dredging into a clean plate or wide bowl.
In another bowl, pour in the tempura batter mix. Add in 2 cups of ice cold water. Whisk together just until mostly incorporated, a few lumps remaining is fine. Do not overmix. *Note: Another way to test your oil in the absence of an instant-read thermometer: gently drip a small amount of batter into the pot. Flakes should float immediately and be golden brown in around 10-15 seconds.
Take each piece of shrimp and coat in flour, making sure to coat the entire piece. Shake off excess flour.
Transfer shrimp into the bowl of tempura batter and coat thoroughly. Let excess batter drip off.
Holding on to the head of the shrimp, gently dip tail first into oil and let it fry for around 3 seconds before gently dropping the rest of the shrimp.
Let it fry until golden brown, for around 2.5-3 minutes. If desired, gently drip more batter on top of the shrimp while frying for extra crunchy flakes. Be careful of oil splatter.
Transfer to a plate lined with tissue to absorb the excess oil. Generously sprinkle with nori powder while hot.
Heat up tempura sauce in the microwave until warmed through, for around 30 seconds. Add grated ginger and radish, if desired.
Serve hot with sauces on the side. Itadakimasu!