Chill the 5 tbsp water and 3 tbsp carbonated water in the fridge. Sift 15 g cornstarch and 75 g unbleached cake flour into a bowl and place in the freezer before you start the recipe (about 30 minutes).
Pour about 5cm (2 inches) cooking oil into a 24cm frying pot and heat to 180 °C (356 °F). Once the oil is almost ready, pour the chilled water and carbonated water into a mixing bowl, and add 1 egg yolk. Whisk until combined.
Add the chilled flour and starch mixture in 3 additions, drawing crosses with chopsticks to combine them rather than mixing. Do not overmix, it is okay if the batter has lumps.
Drop in 2 ice cubes to keep it cold.
Run a knife lengthwise all the way around 2 avocados and twist the halves apart. Remove the pits and cut each half lengthwise into 4 wedges, about 1.5cm thick.
Peel off the skin of each wedge and brush with unbleached cake flour all over.
Once the oil is ready, dip the dusted avocado wedges in the batter and place them in the oil. Fry in batches to avoid overcrowding the pot, I aim for 4-6 wedges per batch in a 24cm pot.
Once they float, turn them over. Fry about 1 minute on each side, 2 minutes in total, and lift them out at pale gold, when the bubbles turn fine and the sound gets lighter.
Transfer to a wire rack in a single layer to drain the excess oil and sprinkle with salt while they're hot.
Serve immediately and enjoy with salt or tentsuyu for dipping!
Notes
Do not judge ripeness by skin color: Press gently beside the stem end and use a fruit that gives just a little. Bacon and Fuerte stay green even when ripe. Leave a rock-hard one at room temperature until it softens.1.5 cm is the line for a creamy middle: Heat reaches the center by the square of the thickness. At 2 cm nothing warms through in about 2 minutes, and anything thinner falls apart in the oil.Never skip the dusting flour: A cut avocado face is slick with fat and batter slides off it. The flour pulls in the surface moisture and turns into a glue layer. Without it the batter peels away and the flesh collapses in the oil.Pale gold is the ceiling: Salt is the only finish here, so there is nothing to hide the bitterness of an over-fried piece. Once the bubbles turn fine and the sound lightens, lift them out ahead of the clock.Fry these last: A crisp batter holds for only a few minutes, and the warm fat in the flesh keeps feeding moisture into it. Finish everything else first, then heat the oil and serve each batch as it comes out.