Butter an 8-inch square pan and line with parchment paper, using enough so that there is some overhang on the edges.
Combine the sugar, cocoa powder, evaporated milk and corn syrup in a large saucepan over medium heat. Cook, stirring constantly, until the sugar dissolves.
Add 2 tablespoons of the butter and stir until the butter melts. Allow the mixture to come to a boil, then cover and boil for 3 minutes.
Remove the lid and continue to cook, without stirring, until the mixture reaches 234 degrees F on a candy thermometer (soft ball stage). Remove from the heat and, without stirring, add the remaining butter, peanut butter and vanilla. Allow the mixture to sit for 10 minutes.
Using a wooden spoon, beat the fudge vigorously until it thickens and begins to lose its gloss (can take anywhere from 5 to 15 minutes). You can also transfer the mixture to the bowl of a stand mixer (or use a hand mixer) and beat it on medium speed, being careful to only mix until it thickens and loses its gloss. Do not overmix.
Immediately pour the mixture into the prepared pan. Allow the fudge to come to room temperature, then cover the pan with plastic wrap and chill it until set. Lift the fudge out of the pan and cut into 1-inch squares. The fudge can be stored in an airtight container at cool room temperature or in the refrigerator.
Nutritional values are based on one piece of fudge