Line a standard loaf tin (23cm by 13cm) with greaseproof paper, then use a flavourless oil to grease the greaseproof paper. This will make removing the caramel easier when it has set.
Add the caster sugar to a heavy bottomed pan and heat the sugar until it starts to turn golden.
Whilst the sugar is cooking bring the cream to a boil in another pan.
When the caramel has reached the correct colour and all the sugar has dissolved, remove the sugar from the heat and add half the warm cream.
The caramel will bubble up and a lot of steam will come out, keep your hands out of the way as the escaping steam will be hot.
When the bubbles subside, add the rest of the cream and again when it has finished bubbling add the butter and the salt and stir.
Return the pan to the heat and cook the caramel until it reaches 122C, then pour the caramel into the pan and leave to set for at least 4 hours.
When the caramel has set, turn it out from the pan, cut the caramel into portions with a sharp knife and sprinkle the top with a little of the sea salt mixed with pepper. Then wrap the caramels in greaseproof paper and store them in an airtight container.
They will keep for about a week like this.