To make the mayo, mix the wasabi powder with 1 tbsp water to form a paste.
Stir in the mayonnaise and sugar, then cover and chill.
In a bowl mix all flours together, gently add the iced liquid, briefly stir (do not stir too much).
Heat the oil in a deep saucepan until a piece of bread browns in it after about 15 seconds. Line a tray with kitchen paper. Dip prawns into batter one at a time and gently lower into the hot oil, you can add a few at a time, allow to cook until the batter is golden and the prawns are pink. Lift onto the kitchen paper to drain, then sprinkle with salt. Serve hot, ready to dunk into the dip.