01 - Heat vegetable oil in a deep pot or fryer to 350°F (175°C). Maintain temperature throughout frying for consistent results.
02 - Whisk together flour, cornstarch, sugar, baking powder, and salt in a mixing bowl until well combined.
03 - Whisk egg and cold sparkling water in separate bowl until combined. Pour into dry ingredients and whisk until smooth, slightly thick batter forms.
04 - Pat marshmallows dry if damp. Using fork or skewer, dip each marshmallow into batter, ensuring complete coating. Allow excess to drip off.
05 - Carefully lower coated marshmallows into hot oil. Fry in batches without overcrowding, cooking 1–2 minutes and turning occasionally until golden brown and crisp.
06 - Remove marshmallows with slotted spoon and drain on paper towels. Serve immediately, dusted with powdered sugar and drizzled with sauce if desired.