In a large heavy based saucepan, add the water and sugar.
Heat on a low-medium heat, and stir constantly until the sugar has dissolved into the water. The water will still look a touch cloudy when dissolved, but there will be no sugar granules any more.
Once the mixture has dissolved together, and it starts to bubble slightly, turn the heat up higher and STOP STIRING.
Leave the mixture alone, and do not touch it. Wait for the mixture to boil away to an amber colour. This often takes between 5-10 minutes, depending on your pan, and hob etc.
Once the mixture has turned to the correct colour, take it off the heat and pour in the cream and whisk in, and add the butter and whisk again (Or, add the butter and then the cream). The mixture will bubble quite a lot but whisk well.
Add in the vanilla, and again whisk. Finally, add in the salt and whisk again.
Once all combined, pour into some clean, and appropriate containers such as jars, and leave to cool on the side. Once completely cool, refrigerate.
Notes
You really need to keep an eye on the caramel when its turning to the amber colour. There are seconds between it turning from a delicious amber colour, to burnt.
It can take ages for it to turn to a different colour, but thats just the process it takes. It's basically heating up, and once it reaches a certain point, it'll turn to the caramel.
The caramel should be kept in the fridge, but it will get harder in the fridge.Microwave to soften what you need.
It'll last in the fridge for up to two weeks as long as the container its put into is spotlessly clean.