In a small saucepan, stir together vinegar, sugar and salt and bring to a boil
Boil vigorously until volume is reduced by half (to 1/2cup)
Reduction should take between 7-12 minutes, but this is only a guideline. Watch closely for best results.
When reduced, stir in butter
Allow to cool.
Store at room temperature or refrigerate. If refrigerated, allow to come to room temperature before using, as butter will harden in fridge