Pop your butter into a saucepan over a low heat.
Allow it to slowly melt, while stirring occasionally.
When the butter has completely melted, add the brown sugar and the cream.
Increase the temperature to medium.
Stir occasionally until the sugar has all dissolved.
Add the salt and give it a quick taste - add extra salt if required (be careful because it will be hot!).
Stir occasionally until the sauce begins to gently simmer and then allow to simmer for 2 minutes (still giving it the occasional stir).
Remove from the heat and set aside.
Store in a sterilised jar for up to 2 weeks.