In a saucepan put all the ingredients the semi-sweet chocolate, the sweetened condensed milk, the butter and optionally the vanilla and the salt.
Heat over medium heat while stirring with a spatula until well melted and smooth.
Pour the fudge mixture into a pan lined with parchment paper, smooth the surface and place in the refrigerator for at least 2 hours.
When the chocolate fudge is completely cold, remove it from the pan and cut it into small squares with a large knife.
Sprinkle with some fleur de sel or crushed nuts and enjoy!