In a pot over a medium heat heat the sugar, stirring constantly with a spatula or wooden spoon. The sugar will start to clump together and after about 6-7 minutes it will melt completely into a thick amber liquid. Be careful not to let the sugar burn.
Once the sugar has melted, add the butter. The mixture will bubble and may splatter a little so be careful as the sugar is extremely hot.
Continue to stir until the butter is melted (1-2 minutes) and then slowly pour in the cream. There will be more bubbling and splattering in this step so again, be careful.
Allow the sauce to come to the boil, and then remove from heat and stir in the salt.
Let the sauce cool in the saucepan before transferring to a jar to store in the fridge.
The caramel will harden as it cools so you will need to heat it before using.