Add balsamic vinegar to a small saucepan over medium heat and bring to a gentle boil. (If you want glaze to be sweeter, add Besti or other sweetener that dissolves easily to the saucepan when you add the balsamic vinegar.)
Reduce heat to medium-low and simmer for about 20 minutes, until volume reduces by half and it coats the back of a spoon. (It will thicken more as it cools.)