Combine sugar, butter, salt and condensed milk in a large saucepan over a medium heat. Allow to boil for 6 1/2 minutes, stirring constantly. Place vanilla essence, chocolate chips, chopped chocolate and the marshmallows into a large mixing bowl.
Pour the hot mixture over the ingredients in the mixing bowl and beat until all the chocolate has melted.
Pour into the already prepared baking tin and gently spread with a rubber spatula.
Allow fudge to sit for 3 hours, probably best to put it into the fridge and then cut into squares. The bigger the better.