Line a 9" square tin with some parchment paper or foil.
Smear lightly with a bit of butter.
Chop the chocolate into small pieces.
Toast the pistachios lightly in a pan. Cut the pistachios into halves.
Put the chocolate, condensed milk and butter into a heavy bottomed pan.
Melt the chocolate over low heat stirring the mixture all the time.
It is done when the mixture comes away from the sides of the pan into a ball.
Add the nuts and mix well. Take the pan off the heat.
Pour the mixture into the lined tray.
Smoothen the top with a spatula.
Let the fudge cool. Refrigerate it till it sets.
Cut into square pieces. The size depends on you. A good guidance is 8X8 rows.
Alternately, let the fudge cool down a bit and then take a tablespoonful and roll into balls.
Store it in the fridge in an airtight container.