Put milk, ½ cup of the caster sugar and butter into a medium saucepan and stir over a medium heat until butter has melted. Add flour and stir until combined. Stir over heat for 2 minutes or until mixture forms a smooth ball.
Transfer dough to the large bowl of an electric mixer and spread over the base to cool for 10 minutes. Add one egg and beat until combined. Add remaining egg and beat until smooth. Put remaining caster sugar and cinnamon into square dish. Stir until evenly combined.
Spoon dough mixture into a large piping bag fitted with a 7mm star pipe. Half fill a large saucepan with oil and put over a medium heat. When oil is hot enough (see tip above), squeeze mixture from piping bag to form a 12cm long churro. Hold over oil and use a knife to cut off from nozzle and allow to fall gently into hot oil.
Cook for about 1-2 minutes or until churro is golden and floats. Remove with a mesh scoop and let drain. Toss in cinnamon sugar immediately until evenly coated. Repeat with remaining batter. Serve warm with melted dark chocolate, if desired.