Place the balsamic vinegar into a small saucepan and add the sugar.
Bring to a boil, reduce the heat and simmer for 12 – 15 minutes, stirring occasionally. Reduce the sauce by half - at this stage, it should lightly coat the back of a spoon.
Remove from the heat.
Allow it to cool, then transfer to an airtight container.
This sauce will keep for 3-4 weeks. Store in the fridge.