1/2 cup balsamic vinegar
Add vinegar to a small saucepan and bring to a boil. Reduce to a very low simmer and cook for 10-15 minutes, until liquid reduces by about half and is slightly syrupy. Remove from heat, pour vinegar in a bowl or glass to pour, and set aside to cool and thicken.
Be careful to not let it boil and burn. It is very easy to burn the vinegar, so just keep it at a very low simmer the whole time and be patient.