Mix cake flour, sugar, baking powder, and salt in a bowl.
Add water and butter to a saucepan and bring to a boil.
When boiling, add the dry ingredients to the mixture and stir well to make the dough.
Let the dough cool, then transfer it to a piping bag.
Pipe the dough into preheated oil at 180℃ and fry until golden brown.
Remove excess oil, then coat the churros evenly with a cinnamon-sugar mix.