Prepare shrimp by taking off shell and deveining. Make 4 slits down the stomach. Flip shrimp on the side and diagonally slice 4 more slits on each side. (This will keep the shrimp straight when cooked.).
In a medium bowl, beat egg and soda water.
Heat a pot of oil to 350˚F/180˚C.
Then add cake flour and mix, but keep it clumpy. Do NOT over-mix.
Dredge shrimp up to the tail and carefully place into frying oil.
Fry for 2-3 minutes or until shrimp is fully cooked.
Place tempura on paper towel to absorb oil.
For the dipping sauce, combine soy sauce and mirin. If using, dissolve dashi in the hot water and add to the soy sauce and mirin.