In a medium saucepan, combine the water, unsalted butter, salt, and ground cinnamon. Bring to a rolling boil over medium heat, stirring occasionally.
Once boiling, add the all-purpose flour all at once, stirring vigorously until the mixture forms a ball and pulls away from the sides of the pan (about 1-2 minutes).
Remove from heat and allow to cool for a few minutes. Add the eggs one at a time, mixing well after each addition until smooth and glossy. Mix in the vanilla extract.
Heat vegetable oil in a deep saucepan or fryer over medium-high heat until it reaches 360°F (182°C). Test readiness by dropping a small piece of dough into the oil.
Transfer the dough to a piping bag fitted with a large star tip. Pipe strips of dough into the hot oil, cutting to desired length with scissors. Fry in batches.
Fry for 2-3 minutes per side, or until golden brown and crisp. Remove with a slotted spoon and place on a wire rack or paper towels.
In a shallow bowl, combine sugar and remaining ground cinnamon. While churros are warm, roll in the cinnamon-sugar mixture until fully coated.