Step 1

Add the condensed milk, chocolate and butter to a medium heavy based saucepan over low heat.

Step 2

Stir until fully melted then continue to cook over low, stirring often until it thickens and bubbles just start to appear - 5-6 minutes.

Step 3

Stir in the vanilla and salt (if using), then pour into the baking tin.

Step 4

Let it sit at room temperature for ½ an hour, then cover with plastic wrap and chill for 2-3 hours until firm.