In a large, heavy bottomed saucepan, mix together cocoa and sugar. Add in the butter, evaporated milk and chocolate.
Heat on low, stirring constantly, until the chocolate and butter is fully melted and the sugar has dissolved.
Simmer on a medium-low heat, stirring occasionally until the temperature reaches a 116 degrees
Remove from heat and cool for 10 minutes.
Line a square slice pan with baking paper.
Once the 10 minutes are up, add in the vanilla and beat with a wooden spoon constantly for 5-10 minutes until the fudge is thick and no longer shiny
Pour into a lined baking tray approx 20x20cm. Allow to set fully before slicing.