Pour 1 cup balsamic vinegar into a saucepan and add 1/2 Tbsp honey (if using).
Bring to a low boil, then reduce heat to keep at a simmer for 12-15 minutes.* Stir occasionally until reduced by half and lightly coats the back of a spoon. It will thicken more as it cools. Transfer to a mason jar or air-tight container, let cool to room temperature then refrigerate for 3 to 4 weeks.
*Reduction time depends on the surface area of your saucepan - a larger saucepan will reduce faster.