Put the sugar into a heavy-based saucepan and set over a medium heat to allow the sugar to gradually melt. As it melts you can gently swirl the pan a couple of times to ensure the caramel cooks evenly, but do not stir it
Once the sugar has melted, the caramel will begin to darken. Continue to let it cook until it reaches a lovely deep amber colour (but keep a close eye on the caramel as it can burn easily)
Add the butter and pour in the cream, taking care as it can bubble up and is extremely hot. Mix with a small handheld whisk until everything is fully incorporated
Put the pan back on the heat and cook, stirring constantly, for a further 2 minutes, until all lumps have dissolved
Take off the heat and stir in the salt and vanilla extract
Mince the raisins finely and stir into the caramel along with the rum (vary the amount of rum depending on your personal preferences - 4 teaspoon will give a moderate alcoholic taste. Try 3 teaspoon for a milder flavour or 5-6 teaspoon for a stronger kick
Allow to cool in the pan for 20 minutes, then carefully pour the caramel into a jar
Let cool completely, then seal and store in the fridge for up to 3 weeks
Bring the jar out of the fridge several hours prior to serving to allow it to soften (do not be tempted to use the microwave on it). Give it a good stir and serve