Pour the balsamic vinegar (and brown sugar if using) into a medium saucepan over medium heat and bring the balsamic vinegar to a gentle boil.
Reduce the heat to medium-low and allow the vinegar to simmer slowly for about 15-20 minutes, stirring occasionally with a heat resistant spoon. As it simmers, the balsamic vinegar will reduce and thicken, forming a syrupy consistency. Keep an eye on it to prevent it from burning or boiling over.
To test the consistency of the glaze, dip the spoon into the saucepan and lift it out. The glaze should coat the back of the spoon and slowly drip off. If it's too thin, continue simmering for a few more minutes. If it's too thick, you can add a splash of water or more balsamic vinegar to thin it out.
Once the glaze has reached your desired consistency, remove the saucepan from the heat and let it cool for a few minutes.
Transfer the glaze to a clean, airtight container and let it cool completely. As it cools, it will thicken slightly more. Makes ½ cup.