In a medium saucepan, combine the butter, sugar and cream. Bring to the boil, then reduce the heat slightly and simmer for 5 - 7 minutes, until thickened. Remove from the heat and stir in the salt and vanilla.
Allow to cool then transfer to an airtight container. Will keep in the fridge for a couple of weeks.
Recipe by thelittleloaf at http://www.thelittleloaf.com/2017/07/03/butterscotch-sauce/