1. Grease an 8-inch (20 cm) square pan and line the bottom and sides with parchment paper.
2. Melt the chocolate and butter in a metal bowl placed over a pot with an inch (2.5 cm) of barely simmering water, stirring until melted. Remove from the heat and set aside.
3. Place the sugar, water and corn syrup in a small saucepot and bring to a full boil over high heat. Immediately remove from the heat and whisk into the condensed milk (make sure the condensed milk is in a heatproof bowl). Whisk in the chocolate, vanilla and salt and let sit for 5 minutes without stirring. Stir the fudge vigorously by hand with a spatula or wooden spoon for a few seconds, then pour into the prepared pan and chill for 2 hours (the fudge slices best when chilled.)
4. Turn the fudge out onto a cutting board and cut into squares. Roll these in icing sugar to coat on all sides and place the fudge portions into mini paper cups if you wish.
5. Once cut, the fudge can be stored in an airtight container at room temperature.