Mix the flour, baking powder, salt, and baking soda in a large bowl.
In another bowl, whisk the egg and cold water together and gradually mix this into the dry ingredients until it comes together. Mix until a smooth dough is obtained, cover with plastic wrap, and leave in a humid place for 1 1/2 hours.
Knead the dough until smooth, then let it rest again for an additional 45 minutes.
In a pot, heat the frying oil to 350˚F (180˚C).
Meanwhile, on an oiled work surface, roll out the dough to form a rectangle about 1/2 inch (1 centimeter) thick, then cut it into 1 1/4 x 3 1/2 inch (3 x 9 centimeter) strips.
Stack two strips on top of each other and press down firmly all the way down the length using a chopstick.
Holding both ends, carefully drop each youtiao into the hot oil.
Turn constantly for 30 seconds so that they cook evenly on all sides, then remove and transfer to paper towels to drain excess oil.
These are delicious served with hot or cold soy milk.