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 until combined. Add to dry ingredients and whisk until smooth and slightly thick.
04 - Pat marshmallows dry. Using a fork or skewer, dip each marshmallow into batter ensuring full coating.
05 - Carefully lower coated marshmallows into hot oil. Fry in batches 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.