Add 1 cup balsamic vinegar and 2 tablespoons dark brown sugar to a small saucepan over medium heat. Bring to a simmer.
Once the mixture begins to bubble, reduce the heat to medium-low heat and allow to simmer until reduced by half, about 10 minutes, stirring occasionally.
Remove from the heat and set aside to cool before serving. Store in an airtight container, like a jar or bottle, in the fridge for up to one month.