01 - Pour vegetable oil into a deep pot or fryer and heat to 350°F.
02 - Whisk together flour, cornstarch, sugar, baking powder, and salt in a mixing bowl until well combined.
03 - In a separate bowl, whisk the egg and sparkling water until combined. Pour into the dry ingredients and whisk until smooth and slightly thick.
04 - Pat marshmallows dry if needed. Using a fork or skewer, dip each marshmallow into the batter, ensuring full coverage.
05 - Carefully lower coated marshmallows into hot oil, frying in batches. Cook 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.