How you reduce balsamic vinegar can make a huge difference in the end result. It's best reduced ever-so gently in an shallow, non reactive oven-proof pan or dish for several hours, 4-5 would probably do the trick, at around 175 F or less, so that it never boils or simmers, then there will be magic. The violent reduction of balsamic over flame can create bitter or harsh flavours as the already caramelized grape must sugar will probably begin to burn.
Using cornstarch to thicken it is always an option but since it's used more frequently to dupe consumers in to believing a product is aged, it’s therefore kinda bogus in our opinion. The point of a real balsamic reduction is to intensify the lovely rich, dried fruit characteristic inherent in a Solera aged authentic balsamic which include notes of fig, raisin, cherry, etc, while creating an unctuous condiment. Thickeners just affect thickness, of course, and do not truly make a product rich or more complex.