In a medium bowl, mix mirin and salt. Place prawns into the mixture. Cover and marinate in the refrigerator at least 20 minutes. Heat oil in deep-fryer or large wok to 190 degrees C.
In a medium bowl, mix together plain flour, ice water, cornflour, egg yolk, salt, white sugar, butter and baking powder.
One at a time, dip prawns into the flour mixture to coat. Carefully place a few prawns at a time in the hot oil. Deep fry until golden brown on all sides, about 1 1/2 minutes. Use a slotted spoon to remove from oil. Drain on paper towels. Serve warm.