Measure out 75 ml cold water into a jug and place it in the fridge for 20-30 minutes. Place sparkling water in the fridge too. (Preferably an unopened bottle.)
Sift 15 g cornstarch and 75 g cake flour into a bowl. Mix them together and place the bowl in the fridge for 20-30 minutes.
While your tempura batter ingredients are chilling, cut 250 g eggplant either into thin slices using a mandoline slicer, thick rounds, or fans (see next step for instruction).
Suehiro/Ogi Cut: Cut off the top of the eggplant and cut it in half lengthways. Cut each piece in half horizontally, and again vertically if the eggplant is particularly big. Cut 5mm slices on each piece while leaving 1cm at the top to hold it together. Gently press to fan out the cuts.
If using suehiro cut or thick rounds, soak the eggplant in cold water with a sprinkle of salt for 5-10 minutes. If using thin slices, sprinkle the surface with salt.
Once your batter ingredients have been chilling in the fridge for 30 minutes, start preheating your oil to 175 °C (347 °F).
In a bowl (or jug), add a few tbsp of the chilled water and ½ tbsp Japanese mayonnaise. Mix until well combined to loosen up the mayo, then pour the rest of the chilled water and 50 ml carbonated 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, it's fine for tempura batter to have lumps.)
Add 2 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. Alternatively, check the temperature with a thermometer. Once the oil is hot, dry the eggplant pieces with kitchen paper and dust them with a thin layer of flour.
Dip them in the batter and then carefully place them into the oil. Make sure not to overfill the pot. (Keep the batter in the fridge between batches.) For suehiro cut or thick rounds, deep fry for about 1 minute on each side. For thin slices, fry for 30 seconds on each side or until crispy.
Once cooked, place on a wire rack to allow the excess oil to drip off.
Enjoy eggplant tempura with tentsuyu sauce or dipped in salt.Eggplant tempura can be eaten as a side or served on top of udon noodles, soba noodles or on top of rice (tendon/tempura donburi).