Add the butter, sugar and salt to a medium-large heavy based saucepan
On a low-medium heat, heat the mixture and stir occasionally until the butter has melted and everything is mixed together
Turn up the heat to medium-high
Stir the mixture (ideally with a whisk/spatula) every 10-15 seconds - you do not need to stir constantly
It will boil and bubble and gradually darken
Keep boiling until it smooths out to a sauce like consistency with a dark amber colour (the colour in the photos)
If you want to use a thermometer, it will need to reach 149ºc
Pour onto a large lined tray immediately
Leave for 20-30 minutes to fully harden
Smash up into pieces
Notes
Once the hard toffee has hardened, you can break it into the shards that you want to serve it as and then it needs storing in an airtight container. It needs to be kept in a cool and dry place, away from direct sunlight and moisture as this can make it sticky.