Place balsamic vinegar in a small non-reactive saucepan over the lowest possible setting on your stove. If needed, use a heat diffuser to reduce heat so that the vinegar warms to about 150°F (do not boil).

Let vinegar reduce, swirling the pan from time to time, until it's thick and syrupy, and reduced to about 1/4 to 1/3 the original amount, about 45 to 60 minutes.

Store in a sealed container.