Grease a rectangular metal tray/ container with butter. Then line it with a greased butter paper - arranged vertically as well as horizontally – to enable easy prying out.
Place a non-stick pan on the stove, pour in the condensed milk placing it on very low heat.
Then add the dark chocolate (broken roughly into bits) and melt while continually stirring the mixture. It takes around 5 minutes to be well incorporated. (My trick is to break the chocolate inside its wrapper itself, to avoid using a hammer :-) to break it or have it fly all over the kitchen table)
Pour the chocolate-condensed milk mixture into the pre-greased pan. Give the tray a little jig so that the mixture is evenly levelled.
Set aside till cooled, then refrigerate it for a couple of hours, or more if not set.
Remove from the fridge, tug gently at the butter paper and voila – there emerges the slab of fudge. Chop it into square bits, into the sizes of your choice.