Line a dish with baking paper (this will ensure for an easier release once set).
Bring a medium or large saucepan to a low heat, add water and sugar. Stir continuously until the sugar has completely dissolved, about 10-15 minutes. Note: I prefer using a non-stick pot, it helps the sugars not stick unnecessarily but not a must. Very import to do this on a LOW heat - patience is required for this recipe.
Stir in the butter and syrup, and continue to mix until the butter is melted and well combined, about 2 minutes. Note: some of the sugar will start sticking to the sides of the saucepan and crystalize, just scrape it down as best you can whilst stirring but it should all be dissolved by step 4 do don't worry.
Add the condensed milk to the mixture and bring the mixture to a gentle bubble whilst stirring.
Let the mixture simmer for 30 minutes, stirring regularly. Note: you want the mixture to turn a medium, leaning to dark golden caramel colour and thicken up properly otherwise you'll end up with very soft fudge, even after setting in the fridge.
Once darkened, remove from the heat. Stir in the vanilla essence and salt. Vigorously whisk the mixture using a wooden spoon or electric whisk until smooth and thickened, about 5-10 minutes depending on which utensil you use. Note: you know it's done when you can roll a piece of the fudge into a firm ball using your finger tips.
Pour the fudge mixture into a lined square dish and refrigerate for 4 hours or overnight. Once cooled, cut it into 9 squares and then halve each one so you end up with18 pieces.