Chill the water and carbonated water.
Mix the flour with ¼ cup (30g) of the potato starch and chill for 30 minutes; keep the remaining starch for coating.
Prep all tempura ingredients by washing, cutting, and drying thoroughly.
Heat the oil to 180°C / 356°F.
In a bowl, combine the chilled water and carbonated water, add the egg, and whisk gently without creating foam.
Add the flour mixture in thirds, stirring lightly with chopsticks in a cross motion; don’t overmix and stop in under a minute, lumps are good.
Add 3 ice cubes if you want the batter extra cold.
Test the oil with a drop of batter; it should sizzle and float.
Lightly coat each ingredient in the reserved starch, shake off excess, then dip into the batter.
Fry until crispy and lightly golden, cooking one type at a time.
Drain on a wire rack.
