1. Put the condensed milk, butter, vanilla and sugar in a large, non-stick saucepan (this will make life easier when it comes to cleaning it!). Bring to the boil and stir continuously. Turn the heat down slightly and keep stirring for 10 minutes, being careful as it is very hot. Don't stop stirring or it will burn/stick to the bottom. It should start to reduce and go a little bit darker in colour. Add the salt, and the chocolate (if using).

2. After 10 minutes, take off the heat and drop a teaspoon of the mixture into a little bowl of cold water to test it. If it forms a solid but moveable blob, the mixture is ready. If not, give it a couple more minutes on the heat.

3. Sit the bottom of the saucepan in a sink, tray or bowl of cold water about 2-3cm depth to help it cool down. Stir until it cools down slightly.

4. Spoon the mixture into a lined tin (we used a square brownie tin). Use the back of a spoon to make it smooth and push it into the corners. It should be half an inch thick.

5. Allow to cool - put it in the fridge to speed up the process. We let ours set for a couple of hours. Slice into pieces.