Mix the flour and cornstarch together in a small bowl. Set aside.
Get a cup and fill it 1/3 up with ice, add water (or seltzer) until it's full and set it aside in the freezer.
Cut off the stem side of the mini eggplants, about a 1/4 to 1/2 inch. Discard.
With the eggplant's cut top facing the top, make a cut starting on the right, ¼ inch in, cut through the eggplant, leaving a 1/3 inch not cut near the top so that the little "wing" you just made is attached at the top. Then keep this up until you reach the other side. Now place your palm over the eggplant and press down, lightly at first, so that the "wings" of the eggplant "fan" out and spread out. Make sure the "wings" don't break off if you need to press harder to get them to fan out. Thinner "wings" are better but don't get them too thin or they'll over cook while the top under cooks. Repeat for the other mini eggplants.
Get your oil hot in a medium sauce pan or appropriate pot and heat until 350-365 °F This oil should never get to a smoking stage. If you don't have a thermometer, please test some drops of the tempura batter and watch to see how fast they turn golden. If they are golden before 3 minutes probably too hot. (Because the eggplants will take 2-4 minutes depending on thickness)
Once the oil is to temperature, get out your ice water or seltzer and mix into the flour cornstarch mix. You may need to strain the ice from falling in, also drizzling the water in will allow you to determine the exact amount of water you need. This batter will be thinner then pancake batter, closer to crepe batter. Add the water until appropriately thinned and immediately dip in your eggplants, coat wel, tap once on the side of the bowl to shake excess off and drop in hot oil, away from you so oil doesn't splash. Repeat until pan is full, you may need to do this in a few batches depending on the pan. Always be sure the temperature of the oil is back to 350-365 °F before your next batch.
When golden all over, remove from oil and place on paper towels to absorb excess oil. Serve right away with a dash of kosher salt or any flaky salt.