01 - Pour vegetable oil into a deep pot or fryer and heat to 350°F. Maintain temperature throughout frying for optimal crispiness.
02 - Whisk together flour, cornstarch, sugar, baking powder, and salt in a mixing bowl until evenly combined.
03 - Whisk egg and sparkling water in separate bowl until blended. Pour into dry ingredients and whisk until smooth, slightly thick batter forms.
04 - Pat marshmallows dry if damp. Skewer each marshmallow with fork or skewer, then dip into batter, turning to ensure complete coverage.
05 - Lower coated marshmallows carefully into hot oil. Fry in batches without overcrowding, cooking 1-2 minutes while turning occasionally until golden brown and crisp.
06 - Remove fried marshmallows with slotted spoon and drain on paper towels. Serve immediately while warm, dusted with powdered sugar and drizzled with sauce if desired.