Toast the barley over a fairly high heat until it starts popping and becomes very fragrant. Cool slightly.
Place into the Thermomix bowl and mill 10 sec/speed 8.
Add the water and boil 10 min/90°C/speed 4. Allow to steep in the Thermomix bowl until cooled.
Strain through a nut milk bag and reserve liquid.
Place sugar, butter and 150g of the black barley liquid (which will be quite thick by now) into the Thermomix bowl and cook 15 min/Varoma/speed 4/MC off
Add the salt and vanilla and stir 10 sec/speed 4. Taste it, when cool enough, to make sure you have the salt right. Add more if needed.
Serve with any dessert that needs a caramel sauce, it is awesome over profiteroles, ice cream or custard style desserts.
This will set quite solid in the fridge and will keep for up to 3 weeks. If frozen, you can keep it for up to 3 months.