Pour the cream, yogurt and vinegar into a sterilised jar, put on the lid and shake well. Remove the lid and place a square of clean kitchen paper on top of the jar before sealing with a piece of string or rubber band.

Leave the soured cream overnight in a cool place, but not in the fridge. The next day you should have a thick, spoonable mixture. To serve, stir through the chives and sea salt, if you like. Will keep in the fridge for up to two days.