Combine water, sugar, salt, and butter in a saucepan and bring it to boil over high heat. Stir in the flour and sourdough starter with a wooden spoon. Reduce the heat and stir vigorously until the mixture forms into a ball.

Transfer the dough to a pastry bag with a large tip. mix sugar with cinnamon. Place the mixture on a plate so you can roll the fried churros in it.

Squeeze out the dough in 2-3 inch / 50-75 mm long strips directly into the hot oil. Fry until golden brown, about 2 minutes per side. Transfer the churros to a plate lined with paper towels and let them drain.

As soon as they are cool enough to handle, roll them in the cinnamon-sugar mixture.