
Place the balsamic vinegar and sugar in a pot. Make sure there is enough room in the pot for the syrup to bubble up.
Optional - add any other optional ingredients to infuse and flavor the vinegar.
Heat over medium heat and stir to dissolve the sugar. Keep heating the mix to bring it to a boil.
Lower the heat so you can control the reduction rate of the vinegar. Also make sure to turn on your exhaust fan, since the balsamic vinegar will have a strong aroma as it cooks down (proper ventilation is highly recommended).
Whisk frequently and cook the vinegar until it has reduced by 50% (about 1 cup).
This takes about 20 - 30 minutes for me, but the time will depend on whether you use a larger / smaller pot and the stovetop heat. Keep an eye on the reduction so you can monitor it. If it evaporates too quickly, and you leave it unattended, the syrup could burn.
Turn off the stove and taste the vinegar reduction - make sure to let it cool down before you taste as this will be very hot. You can add more sugar if you like, and if the reduction is too sweet, add a little balsamic vinegar to counter the sweetness.
Allow the syrup to cool down further. If you added any flavoring ingredients, strain the mixture to remove the additions after 20 - 30 minutes of cooling.
Once the vinegar syrup has mostly cooled down, transfer it to a jar or container. Keep it in the fridge to cool down completely.
Use the balsamic syrup while slightly warm, or at room temperature, or chilled.