Pour the vinegar (and sweetener if using) into a small saucepan.
Over medium heat, bring the vinegar to a low boil.
Reduce the heat to medium-low and let the mixture lightly simmer, whisking every couple of minutes to make sure nothing sticks and burns.
When the vinegar has reduced in half, it should coat a spoon in thickness. If you use just the vinegar this will take about fifteen to twenty minutes. If you used a sweetener it will take less time - about eight to ten minutes.
Remove the pan from the heat and let it cool down completely.
Transfer the balsamic glaze to a glass jar with a lid. This will keep stored in a cool, dark spot for up to a month.