Set up. Line a small baking sheet or tray that fits in your freezer with parchment. Mix the sugar and cinnamon in a shallow dish.
Boil the base. In a medium saucepan, bring the water, butter, 1 tablespoon sugar and salt to a full rolling boil over medium-high heat, until the butter has melted completely.
Add the flour. Take the pan off the heat, add all the flour at once and stir hard with a wooden spoon for about 1 minute, until no dry flour remains and a rough ball forms.
Dry the paste. Return the pan to medium-low heat and keep stirring for 1 to 2 minutes. It's ready when the dough pulls cleanly away from the sides, looks like stiff mashed potatoes and leaves a thin film on the bottom of the pan.
Cool slightly. Transfer the dough to a mixing bowl and beat it for 2 to 3 minutes with a hand mixer on low (or a sturdy spoon), until it stops steaming and feels warm, not hot.
Add the eggs. Beat in the eggs one at a time, mixing until each one disappears before adding the next. The dough will look broken at first, then turn smooth. Beat in the vanilla. The finished dough is thick and glossy, and a swipe of the spatula holds its shape; it should not pour or slump.
Pipe. Spoon the dough into a piping bag fitted with a large open star tip (a 1M or a ½-inch open star). Squeeze out any air at the top. Pipe 4-inch logs about ¾ inch wide onto the parchment, about 1 inch apart, snipping each one off with scissors. You should get about 16.
Freeze. Freeze the tray for 30 minutes, until the churros are firm enough to pick up without bending.
Preheat and load. Preheat the air fryer to 375°F (190°C) for 3 to 5 minutes. Cut the parchment into pieces that fit your basket and set 6 to 8 frozen churros on them, 1 inch apart. Spray the tops lightly with cooking spray.
Air fry. Cook for 6 minutes, until the tops are golden. Flip, spray again and cook 5 to 6 minutes more, until deep golden with crisp, defined ridges.
Check the center. Cut one churro from the first batch in half. The inside should be soft and moist but set, with no wet, glossy dough. If it's still wet, cook 2 minutes more at 325°F (165°C).
Coat. Within a minute of taking them out, roll the hot churros in the cinnamon sugar. Repeat with the second batch, keeping it in the freezer until its turn.
Optional dip. Heat the cream until steaming, pour it over the chocolate chips, wait 2 minutes, then stir until smooth.