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