Measure out 200 ml cold water into a jug and place it in the fridge for 20-30 minutes. Place 100 ml sparkling water in the fridge too. (Preferably an unopened bottle.)
Sift 30 g potato starch and 150 g cake flour into a bowl. Mix them and place the bowl in the freezer for 20-30 minutes.
While you wait for the water and flour to chill, prepare your tempura ingredients by washing, cutting, and drying them with kitchen paper to remove any excess moisture.
Once 30 minutes have passed, start preheating your oil to 180 °C (356 °F).
Pour the chilled water and sparkling water into a bowl.
Crack the egg into the bowl and whisk. (If bubbles or foam forms on top, scoop it out with a spoon.)
Add the sifted flour and starch one third at a time, gently drawing crosses through the mixture with the chopsticks to incorporate it. (Do not whisk or over mix.)
Test your oil by adding a drop of batter, if it sizzles and floats you're ready to fry.Coat your ingredients in a thin layer of flour, brush off the excess and then dip them in the batter. Carefully place the battered ingredients straight into the oil and fry until crispy but before they turn golden. (I recommend frying one type of ingredient at a time to ensure even cooking.)
Once cooked, place on a wire rack to allow the excess oil to drip off.
Serve and enjoy!