Prepare the Churro Dough:

In a medium saucepan over medium heat, combine water, sugar, butter, and salt. Bring to a boil, then remove from heat. Add the flour all at once and stir vigorously until the dough forms a ball.

Incorporate Wet Ingredients:

Let the dough cool for 5 minutes. Add the egg and vanilla extract, mixing until smooth.

Pipe and Fry the Churro Rounds:

Heat vegetable oil in a deep frying pan or pot to 375°F (190°C). Transfer the dough to a piping bag fitted with a star tip. Pipe 2-inch rounds directly onto parchment paper or a silicone mat. Carefully transfer the rounds into the hot oil and fry for 2–3 minutes per side until golden brown. Remove and drain on paper towels.

Coat the Churro Rounds:

In a small bowl, mix granulated sugar and cinnamon. Roll the churro rounds in the mixture while they are still warm.

Assemble the S'mores:

Roast the marshmallows over a flame or under a broiler until golden and gooey. Place a piece of chocolate on one churro round, top with a roasted marshmallow, and finish with another churro round to create a sandwich.

Serve and Enjoy:

Serve your Churro S'mores immediately for a warm, melty, and delicious dessert!