Let’s get cooking.
Prepare before frying: mix the 75 g coating sugar and cinnamon in a shallow bowl. Line a plate with kitchen paper. Fit a sturdy piping bag with an open star nozzle about 1 cm wide, and set out scissors and a slotted spoon.
Put the 100 g butter, 120 ml milk, 120 ml water, 1 tbsp caster sugar and salt in a saucepan over medium heat. Stir until the butter melts, then bring just to the boil.
Remove from the heat and add all 125 g flour. Beat firmly with a wooden spoon until there are no dry patches. Return to low heat and stir for about 1 minute, until the dough forms a ball and pulls away from the sides.
Transfer to a mixing bowl and leave for about 5 minutes, until warm rather than very hot. Beat in the 2 eggs a little at a time, fully combining each addition. The mixture may split at first; keep beating until smooth, thick and glossy. Spoon into the piping bag.
Make the chocolate sauce: put the chopped chocolate in a heatproof bowl. Heat the 100 ml cream in a small saucepan until steaming, then pour over the chocolate. Leave for 1 minute and stir from the centre until smooth. Set aside while you fry.
Pour the frying oil into a deep, heavy saucepan, keeping it no more than one-third full and using a pan that gives enough depth for the churros to float. Heat to 175°C, checking with a frying thermometer. Keep the pan attended throughout frying.
Hold the nozzle close to the oil surface and carefully pipe 3 cm lengths, snipping the dough with scissors. Fry a few at a time so they have room to float. Turn with a slotted spoon and cook for about 3–4 minutes until golden on all sides.
Lift one out and check that the centre is set rather than wet dough; cook the batch longer if needed. Drain briefly on kitchen paper, then toss in the cinnamon sugar while warm. Let the oil return to 175°C before the next batch.
Serve the churros freshly cooked with the chocolate sauce. If the sauce has thickened, warm it gently while stirring until it is suitable for dipping.
