Place the balsamic vinegar in a sauce pot and heat over medium high until boiling.
Lower the heat so the vinegar maintains a steady simmer. Simmer for 10-15 minutes, stirring occasionally, until the vinegar is reduced by half. It should be thick, dark in color, and coat the back of the spoon.
Use immediately or cool and store in an airtight container in the fridge for up to 3 months.
Notes
If you like your reduction to be a little sweeter, feel free to add 1-2 teaspoons of sugar to the vinegar before boiling. You can also stir in honey at the end.
This recipe can be doubled if you'd like to have a larger amount on hand.