It's important to have all of your ingredients ready before beginning! Read through the whole recipe before you start and measure out all your ingredients beforehand (the peanut butter, butter, vanilla, and salt should be prepped and ready to go nearby).
Prepare an 8×8 baking dish or 9×5 bread pan by lightly greasing with butter or lining with parchment paper. Set aside.
Combine sugar and evaporated milk in a medium-sized saucepan over medium heat.
Stir ingredients occasionally over medium heat. During this time (only before boiling), use a wet pastry brush to brush down the sides where any sugar may have begun to settle on the side of the pot to prevent sugar crystals.
Continue to stir occasionally until sugar is dissolved and mixture comes to a boil (don't turn up the heat, keep on medium or you run the risk of burning your fudge). Once mixture comes to a boil, attach your candy thermometer. Be sure that the point of the candy thermometer is in the middle of the mixture and not touching the bottom of the pan.
Cook, stirring continuously, until fudge reaches 234-236°F (112-113°C).
Once fudge reaches temperature, immediately remove from heat and add your peanut butter, butter, vanilla extract, and salt. Stir (avoid scraping the sides of the pot) until butter and peanut butter are melted and mixture is smooth and has started to thicken (1-3 minutes).
Pour into prepared pan (don't scrape the sides of the pot while pouring) and allow to set completely (several hours at room temperature or you can expedite the process by refrigerating).
Once fudge has set, slice into small pieces and serve.