Prepare a 20cm by 30cm square tray with non-stick parchment paper or cling film.
Over medium/low heat , pour in a saucepan, the condensed milk, chopped chocolate, butter and a pinch of salt.
Stir and bring to a gentle simmer for 4-5 minutes until fully combined and starting to be bubbly.
Remove from the heat and pour into the lined tray.
Sprinkle extra salt flakes if desired and cool at room temperature for 20 minutes.
Leave to set at room temperature for a few hours or in the fridge for about 2 hours.
Once set, remove from the fridge, cut into squares and store until needed in a tight container.