Tip the rhubarb, white wine vinegar and sugar into a saucepan and bring to the boil. Simmer for 10 mins until the rhubarb is tender. Add the butter and blitz in a blender or with a hand blender until smooth. Different types of rhubarb have a different water content, so if the sauce seems a little soupy, simmer until it has reduced slightly. Can be made up to two days ahead and kept chilled. Reheat before serving.