Mix the sugar and cinnamon on a large plate and set aside. Line tray with baking paper.
Heat the water, sugar, salt, vanilla and butter in a saucepan over medium heat. Bring to the boil then turn heat off and immediately add all the flour.
Stir the flour vigorously with a wooden spoon until the dough forms a ball. Set it aside for a few minutes to cool down a little but pipe while stir warm and pliable.
Fit a large piping bag with a large closed star tip. Spoon the dough into the bag and carefully pipe the hearts on the baking paper. If you mess up put the dough back in the bag and try again!
Heat the oil in a large, deep pot. If you have a candy thermometer clip it on. The oil is ready when it reaches 190C | 375F. (To check if the oil is ready without the use of the thermometer drop a small cube of bread in it. It should brown within a minute.)
Lower the hearts into the oil using a wooden spoon (they keep their shape remarkably well). Do not fry more than 2 at a time as the oil temperature will drop. Use chopstick to flip and fry until golden - about 2-3 minutes on each side.
Drain on kitchen towels then roll in the cinnamon sugar.
To make the chocolate sauce, heat the cream until just boiling then pour it over the chopped chocolate. Let it stand a couple of minutes then stir until chocolate is melted. Stir in the chilli powder if using. Serve the churros plain or dip them in the chocolate sauce.
You can draw hearts on the baking paper as a template. Flip the paper over when you are piping the hearts. You can also pipe in straight lines, or pipe letters etc. If you have a fryer, use it - it makes the process a little easier.