You mean a balsamic reduction?
Basic recipe: combine 1 1/4 cup Balsamic Vinegar and 1/4 cup brown sugar. Bring to a boil then gently reduce until thick and to your desired consistency. Watch the bubbles. As the bubbles start to get larger, the reduction will get thicker and you need to really watch it at that point or else it will burn and/or over-reduce.
"Boldness has genius, power, and magic in it."
Johann Wolfgang Von Goethe