Combine the butter, sugar and 1 cup (250ml) water in a saucepan. Bring to the boil over high heat. Remove from heat. Add flour and stir to combine. Place over high heat and cook, stirring constantly, for 1-2 mins or until dough is smooth and coming away from the side of the pan. Set aside for 10 mins to cool.
To make the chocolate sauce, combine the chocolate, cream and sugar in a small saucepan over medium-low heat. Cook, stirring, until chocolate melts and mixture is smooth.
Combine the extra sugar and cinnamon on a large baking tray.
Transfer the dough to a bowl. Use an electric mixer to beat until smooth. Add the eggs, 1 at a time, beating well after each addition, until mixture is smooth and glossy. Transfer to a piping bag fitted with a 2cm fluted nozzle.
Add enough oil to a large saucepan to reach 6cm up the side of the pan. Heat to 180°C over medium-high heat (when oil is ready, a cube of bread turns golden brown in 15 secs). Pipe three 12cm lengths of dough into the oil, cutting the dough with kitchen scissors. (To make baby churros, cut into 3cm lengths.) Cook for 3 mins or until golden brown. Use a slotted spoon to transfer to a plate lined with paper towel. Add to the cinnamon sugar and toss to coat. Repeat with remaining dough.
Serve churros warm with the chocolate sauce for dipping.