Measure out 100 ml water into a jug and place it in the refrigerator for 30 minutes along with an unopened bottle of sparkling water.
Sift 15 g potato starch and 75 g cake flour into a bowl. Mix them together and place the bowl in the fridge or freezer for 30 minutes.
Place 10 large shrimp in a bowl with 2 pinches salt and 1 tsp cornstarch and rub them over the surface of the prawns.
Once evenly covered, wash the shrimp with cold water to rinse off the salt and corn starch. Dry them with kitchen paper and transfer to a chopping board.
Place the shrimp on its side and with the tail folded in half and trim it diagonally. The pointy tip should be cut off and be the shorter part of the diagonal. (This stops oil from splattering out when it's frying and it looks nicer too!)
Turn the prawn so the belly is facing up, and make diagonal incisions along the whole body. The incisions should be about 1cm apart and go about ½ way through the prawn.
Once 30 minutes have passed, start preheating your oil to 180 °C (356 °F) to 190 °C (374 °F).
In a bowl (or jug), add a few tbsp of the chilled water and ½ tbsp Japanese mayonnaise. Mix until combined, then add the rest of the chilled water and 50 ml sparkling water into the bowl.
Add the flour/starch one-third at a time, gently drawing crosses through the mixture with the chopsticks to incorporate it. (Do not whisk or over-mix.)
Add a couple of ice cubes to the batter and mix to make it extra cold. (optional)
Test your oil by adding a drop of batter, if it sizzles and floats you're ready to fry. (If it sinks, heat more and keep the batter in the fridge while you wait.)Once the oil is hot, dry the shrimp with kitchen paper and dust them with a thin layer of flour. Dip them in the batter and then immediately place them into the oil one by one. Make sure not to overfill the pot. (Keep the batter in the fridge between batches.)
Fry for about 2 minutes or until crispy but not too golden (tempura should be pale in color). Once cooked, place on a wire rack to allow the excess oil to drip off.
Serve and enjoy!