Step 1: Pour the balsamic vinegar into a small, heavy-bottomed saucepan. If using, stir in the optional brown sugar or honey until dissolved.
Step 2: Place the saucepan over medium heat and bring the vinegar to a gentle simmer. You should see small, consistent bubbles around the edges, but avoid a rapid boil.
Step 3: Reduce the heat to low-medium and allow the vinegar to continue simmering gently, uncovered. Stir occasionally to prevent scorching.
Step 4: Continue to simmer for 10-15 minutes, or until the vinegar has reduced by about half and coats the back of a spoon. Remember, it will thicken significantly more as it cools. A good test is to dip a spoon in; if you can draw a clean line through the glaze on the back of the spoon with your finger, it's ready.
Step 5: Remove the saucepan from the heat. If using, stir in a tiny pinch of salt to balance the flavors. Transfer the balsamic glaze to a heatproof jar or airtight container.
Step 6: Let the glaze cool completely to room temperature. As it cools, it will thicken to a beautiful, syrupy consistency. Store in the refrigerator for up to 2-3 weeks.