Combine all the ingredients, apart from the vanilla, in a large, deep saucepan.
Put the pan over a medium heat and simmer, stirring often, until the mixture
turns a dark caramel colour (about 30-45 minutes).
Reduce the heat if it catches.
Add the vanilla bean paste and increase the heat to a boil.
When it reaches 120°C on a sugar thermometer, pour into a big bowl and leave to cool
for a couple of minutes.
Beat the mixture for about 10 minutes with a wooden spoon (or a stand mixer)
until it becomes grainy and it loses its shine.
Spread the mixture evenly over a lined 40cm baking tray and leave for at least 2 hours
at room temperature to cool.
Once completely cool and set, cut into squares to serve.