Combine first three ingredients in a small bowl. Cover the dip and set aside until needed. Set a prawn, curved side up, on a work surface and cover with plastic wrap. Use a kitchen hammer to lightly hit and flatten the meat of the prawn into an elongated shape. Set on a plate, and then repeat with remaining prawns.

Heat the oil in a deep fat fryer to 350 Degrees F. Preheat the oven to 200 Degrees F.

Set a wire-rack over a baking sheet. Combine flour, cornstarch and sugar in a small bowl. Combine the soda water and egg yolk in a second medium bowl. Add the flour mixture and mix until a thin, lump-free batter forms.

Working in batches, dip the prawns, holding on to the tail, into the batter, draining away the excess. Deep-fry until light golden, about 3 to 4 minutes. Set the cooked prawns on the wire rack; keep warm in the oven until the rest are deep-fried. Arrange the tempura on a serving plate with the dip alongside.