Add the balsamic vinegar and brown sugar to the Thermomix bowl. Heat for 20 min/Varoma/speed 1, without the measuring cup.
Notes
After the 20 minutes is up, the glaze will still look very runny. Don't worry, it thickens considerably into a lovely thick glaze within a couple of hours.
Let the glaze cool to room temperature, then store in an airtight container in the fridge for up to a month.
There's no need to use the simmering basket or splatter guard. Let the steam escape as it cooks.
Open a window before you start, as it fills the kitchen with a strong smell while it cooks. You'll also get a fresh hit of fumes when you take the lid off at the end, so be prepared.
The glaze reduces a lot during cooking and it's ready when it coats the back of a spoon.