Add the balsamic vinegar and brown sugar to the Thermomix bowl. Heat for 20 min/Varoma/speed 1, without the measuring cup.
Notes
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. Once it's finished cooking, avoid standing directly over the bowl, as the fumes are strong and quite pungent.
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.
The glaze reduces a lot during cooking and it's ready when it coats the back of a spoon.
Store the balsamic glaze in an airtight container in the fridge for up to a month.