Whisk together all-purpose flour, baking powder, baking soda (if using), salt, and sugar. Add egg and gradually mix in enough water to form a sticky dough. Knead for 7-9 minutes until smooth and elastic. Cover and let rest for 1 hour.

Roll out dough into a rectangle about 1/2 inch thick. Brush with vegetable oil, cover, and rest for another 2 hours. Cut into 3 cm wide strips, layer them and adhere two pieces together for shaping.

Heat vegetable oil in a deep frying pan over medium-high heat to 180-190°C (400°F). Stretch each dough strip slightly and fry in batches for about 1 minute until golden brown.

Remove Youtiao with a slotted spoon and drain on paper towels. Serve warm with soy milk or congee.