1lbbittersweet or semisweet chocolatecoarsely chopped
1/4Cbalsamic vinegar* plus 2 T for brushing on top of the fudgeI used saba, which is a very expensive traditional aged balsamic. However, the best balsamic that you care to spend your money on will do. I will tell you though that saba is amazing and would make a superb gift for any balsamic lover.
Line the bottom and sides of an 8 x 8 x 2 inch pan (20 x 20 x 5 cm) with aluminum foil. Make sure you go up the sides. You can then easily lift the sides to take the fudge out of the pan when it's done.
Combine the condensed milk, chopped chocolate, and butter in a stainless steel bowl and place over a saucepan of simmering water.
Melt the chocolate mixture, stirring frequently. Make sure the mixture doesn't get too hot or the fudge may be grainy.
Stir until smooth (it will be very thick).
Remove from heat and stir in the balsamic.
Spread the fudge into the prepared pan, smoothing the top with an offset spatula.
Let stand at room temperature for a couple of hours.
Using a pastry brush, brush some balsamic on top of the fudge and let sit for another hour or so until the balsamic is a bit less liquidy and simply sticky.
Carefully remove the fudge from the pan by lifting the edges of the foil. With a long, sharp knife, cut the fudge into pieces.
Store in the refrigerator for several days, or freeze, well wrapped, for several months. Let thaw at room temperature, unwrapped, for a few hours before eating.
*Should you not want to add the balsamic, feel free to leave it off and replace with 2 t of vanilla extract.