Use a medium size heavy bottomed saucepan. Pour the entire bottle of balsamic into the saucepan. Add 1/3 cup of granulated sugar to the vinegar. Stir until the sugar has completely dissolved.
Bring the balsamic to a boil, then lower the heat to a simmer. Allow the balsamic to gently bubble and reduce by 2/3 in volume and thicken to the consistency of syrup.
Simmer for 30-40 minutes. Stir periodically to prevent sticking.
After it has reduced by 2/3 in volume test the thickness with a spoon. If the glaze coats the back of the spoon turn the burner off and leave the saucepan on the stove for a few minutes.
After 5-10 minutes remove the pot from the stove and stir.
Allow the glaze to cool for an additional 5-10 minutes away from the heat. The glaze should resemble a thick syrup. You can use it immediately or store refrigerated in an air tight container for up to 1 week. It will continue to thicken as it cools,
Warm after cooling if needed to thin.