Mix the sugar and cinnamon for the churros in a wide, shallow dish: this is for shaking the cooked churros about in, to coat them later.
Melt all the chocolate sauce ingredients in a heavy-based saucepan, really gently, and once the chocolate starts to melt, stir everything together and take off the heat and leave in a warm place.
To make the churros, put the flour into a bowl and stir in the baking powder then beat in the olive oil and 250ml freshly boiled water from a kettle. Keep mixing until you have a warm, sticky dough, and leave to rest for about ten minutes or for as long as it takes for the corn (or vegetable) oil to heat up.
Heat the oil for frying in a smallish saucepan; it should come about a third of the way up the sides of the pan. When you think it's hot enough, toss in a cube of bread and if it sizzles and browns in about 30 seconds, the oil is hot enough; or if you're using an electric deep-fat fryer or otherwise have the means to check the temperature, it should be at 170°C. Keep a watchful eye on your hot oil pan at all times.
When you are ready, load up a piping bag with a large star-shaped nozzle (8mm) and fill with the churros dough. Squeeze short lengths (approx. 4-5cm) of dough into the hot oil, snipping them off with a pair of scissors as you go. I love the squidgy feel of this.
Cook about three or four at a time and, once they turn a rich golden brown, fish them out of the oil with a slotted spoon or spatula or with tongs onto a baking sheet lined with some kitchen roll. To keep the cooked churros warm while you fry the remaining dough, transfer them, after blotting with kitchen roll, to a parchment-lined baking sheet and hold in a low oven (100°C). Even if you let them sit out of the oven, they do need 5-10 minutes to rest before you eat them, to allow them to set inside.
Toss all the hot churros into the sugar and cinnamon and shake them about to get a good covering, just before serving.
Once you have finished making the churros, pour the chocolate sauce into individual pots (to avoid the double-dipping dilemma) and dip'n'dunk away.
Recipe from Nigella: Kitchen by Nigella Lawson (published by Chatto & Windus).