Start heating a pot of cooking oil to 180 °C (356 °F). While you wait, cut 4 chikuwa fishcake in half lengthways, then cut in half again diagonally crosswise.
Take a large bowl and add the 2 tbsp unbleached cake flour, 2 tbsp potato starch (katakuriko) and 1 tbsp aonori (dried green seaweed powder). Crush 1 tsp bonito flakes (katsuobushi) in your hand to make a fine powder and sprinkle it into the bowl. Mix until all the ingredients are evenly distributed.
Pour 60 ml carbonated water into the dry ingredients and gently stir until combined.
Add the chikuwa to the bowl and mix until evenly coated.
Once the oil is hot, add the battered chikuwa and fry for 1 minute on each side.
Transfer to a wire rack and rest for a few minutes to drain any excess oil.