In a medium saucepan, combine the water, butter, sugar, and salt. Heat over medium heat, stirring occasionally, until the butter melts and the mixture comes to a simmer.

1. Add the flour to the saucepan all at once, stirring vigorously until the dough forms and pulls away from the sides of the pan (about 1-2 minutes). Continue cooking for another minute to dry out the dough slightly.
2. Remove from heat and let the dough cool for 5 minutes. Add eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
3. In a small saucepan, heat the heavy cream over medium heat until it simmers. Remove from heat, add the chopped dark chocolate, and stir until smooth. Stir in butter and vanilla extract, then set the sauce aside to keep warm.
4. Heat 3 inches of vegetable oil in a deep pan or large pot to 350°F (175°C). Pipe the dough into 3-4 inch lengths into the hot oil, frying in batches for 2-3 minutes until golden brown and crispy.
5. Use tongs or a slotted spoon to remove churros and drain them on paper towels. Roll the churros in a cinnamon sugar mixture while still warm.
6. Serve immediately with the warm chocolate dipping sauce on the side. Enjoy!