In a medium saucepan, heat oil until it registers 350 degrees F on a thermometer. In a medium bowl, whisk together 1 cup flour, 1/2 cup cornstarch, Panko, and baking powder. Whisk in soda water.
In a separate medium bowl, combine remaining 1/2 cup flour and 1/2 cup cornstarch. Working with one chocolate bar at a time, dip the chocolate in the batter, then the dry flour mixture, and then dip in batter again. Transfer the battered chocolate bar to the hot oil and fry, turning occasionally, until evenly golden, 4 to 5 minutes. Repeat with the remaining chocolate bars.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.