01 - Pour vegetable oil into a deep pot or fryer and heat to 350°F.
02 - In a mixing bowl, whisk together the flour, cornstarch, sugar, baking powder, and salt until evenly combined.
03 - Whisk the egg and sparkling water in a separate bowl until blended. Pour into the dry ingredients and whisk until smooth and slightly thick.
04 - Pat the marshmallows dry. Using a fork or skewer, dip each marshmallow into the batter, turning to coat completely.
05 - Carefully lower the coated marshmallows into the hot oil in batches. Fry for 1–2 minutes, turning occasionally, until golden brown and crispy.
06 - Remove with a slotted spoon and drain on paper towels. Serve immediately, dusted with powdered sugar and drizzled with sauce if desired.