1. Preheat pizzelle iron.
2. Weigh or lightly spoon potato starch and flours into a dry measuring cup; level with a knife. Combine potato starch, flours, granulated sugar, and next 3 ingredients (through salt) in a medium bowl, stirring with a whisk.
3. Combine 1/4 cup warm water and next 5 ingredients (through egg white) in a large bowl; stir with a whisk. Slowly add flour mixture to egg mixture; stir with a whisk until smooth.
4. Coat pizzelle iron with cooking spray. Spoon 1 tablespoon batter onto hot iron, spreading batter to edges. Cook according to manufacturer’s instructions. Repeat procedure with remaining batter. Top pizzelles with powdered sugar.
Cannoli Shell Variation: Remove hot pizzelle from iron; wrap around a cannoli mold or the handle of a large wooden spoon, overlapping edges of shell. Remove shell from mold; cool on a wire rack.