Place all of the ingredients in a large saucepan and bring to the boil over medium heat, stirring constantly to stop the mixture sticking to the saucepan.
Lower the temperature and allow the mixture to not simmer but boil at a lower temperature; continue stirring in a circular motion.
Keep stirring for 20 minutes and eventually the mixture should turn a dark caramel colour and about double its original thickness. Do not be alarmed at the brown streaks, that is just the brown sugar doing its magic!
Remove from the heat and mix with an electric hand mixer for about 7 minutes or so until it has thickened a fair bit and is beginning to set on the sides of the saucepan, but don’t mix it so much that it gets very tough.
Pour into a cake tin or small baking tray, smooth out, and after 10 minutes score into squares with a small knife so it is easy to break when it has fully set after about 4 hours.