Combine flour and corn starch in a medium bowl. Gradually stir in 1/2 cup water and soy sauce until smooth. Stir in additional water, 1 tablespoon at a time, until batter is just thick enough to coat shrimp (about the consistency of very heavy cream).
Pour oil into 2-quart saucepan, filling no more than 1/3 full. Heat over medium heat to 350°F.
Stir 1 tablespoon of the hot oil into batter. Stir in baking powder.
Coat shrimp with batter. Carefully add shrimp, a few at a time to hot oil; fry 2 to 3 minutes or until golden brown. Drain on paper towels.