In a heavy-bottomed sauce pan, bring Balsamic vinegar to a low boil (about a medium heat) and add the sugar.
Whisk constantly to prevent burning--about 15 minutes.
This will reduce to about a 1/2 cup of thick syrup.
Recipe Notes
The longer you reduce, the thicker your sauce will be, producing a smaller yield, conversely, the shorter the reduction, the thinner the sauce, giving you more, about 2/3 cup.