01 - Heat vegetable oil in a deep pot or fryer to 350°F.
02 - In a mixing bowl, whisk together the flour, cornstarch, sugar, baking powder, and salt.
03 - In a separate bowl, whisk the egg and sparkling water until combined. Add to the dry ingredients and whisk until you have a smooth, slightly thick batter.
04 - Pat the marshmallows dry if needed. Using a fork or skewer, dip each marshmallow into the batter, ensuring it is fully coated.
05 - Carefully lower the coated marshmallows into the hot oil, frying in batches to avoid overcrowding. Fry for 1-2 minutes, turning occasionally, until golden brown and crisp.
06 - Remove with a slotted spoon and drain on paper towels. Serve immediately, dusted with powdered sugar and drizzled with sauce if desired.