Have a parchment paper handy, placed on a greased pan.
To make the evaporated milk, boil the milk in a thick bottom pan, constantly stirring to reduce the milk to 1 cup.
Then, in the same pan, add the condensed milk, sieve the cocoa powder, and add butter. Cook till it forms a thick mass, and when you stir, the mixture should leave the sides. It takes about 20 - 25 mins.
When the fudge is ready, pour over the prepared sheet. Press on all sides to form even shape. Refrigerate for about 2 hours. Cut into bite size pieces.