Place the sugar in a large heavy-bottomed pan over a medium heat. Once the edges of the sugar start to turn to liquid, stir with a heat-resistant rubber spatula. Keep stirring every few seconds.
The sugar will initially go very lumpy, then will turn to a golden liquid. Continue stirring until all the sugar (including any smaller lumps) has melted. If it seems to be taking a long time, turn the heat down while stirring to prevent the caramel from burning.
Once all the sugar has melted, very carefully add the butter to the pan. It will bubble and spit, so you may want to lower it in with a spoon to be extra careful.
Stir the mixture for 2 minutes. You may find that the butter doesn't completely stir in - some of it may stay on top. This is fine. It will mix in properly once the cream is added.
Pour in the cream while continuing to stir. The caramel will bubble up quite a lot at this point. Once combined, allow the caramel to bubble for a minute - stirring once or twice - then turn off the heat.
Leave to cool, then pour the caramel into one large or two small sterilised jars.
Serve once cooled or cover and refrigerate until needed.