Line an 8-inch tin with parchment paper, making sure the sides are fully covered and there is some overhang.
Combine the sugar, cocoa powder, evaporated milk and corn syrup in a large heavy bottomed saucepan over a medium heat. Stir constantly until all the sugar has dissolved.
Add 2 tablespoons of the butter and stir in until completely melted in. Allow the mixture to come to the boil and cover and boil for 3 minutes.
Remove the lid and continue to cook, without stirring, until the mixture reaches 234F / 112C on your thermometer.
Remove from the heat and without stirring add the remaining butter, peanut butter and the vanilla extract and allow the mixture to sit for 10 minutes.
After 10 minutes, using a wooden spoon beat the mixture vigorously until it thickens and begins to loose its shiny gloss. This process can take anything from 5 to 15 minutes. You can use a stand mixer but be careful not to over mix the fudge, keep a close eye on it.
Immediately pour the fudge into the prepared tin. Allow the fudge to cool to room temperature, then cover the tin with plastic wrap and chill until set about an hour.
Lift the fudge out of the pan and cut into 1 inch square pieces.
The fudge can be stored in an airtight container in a cool room or in the fridge for up to 10 days. Not that mine ever lasts that long!