Perfect for gifts and during the holiday season, this soft, chewy fudge doesn't use condensed milk, marshmallows or corn syrup to have a deliciously craveable texture and flavour.
In a medium saucepan, add the brown sugar, chocolate, 75g (5 tbsp) of butter, the milk, cocoa powder and salt. Set over medium heat and, stirring constantly, gently melt the butter and chocolate and bring the mixture up to a boil.
Continue stirring constantly to avoid scorching on the bottom of the pan. Cook the syrup until it registers between 112-114°C (234-237°F) on an instant-read thermometer - or a candy thermometer.
Immediately pour the syrup into a heatproof bowl (or the bowl of a stand mixer) - ensuring not to stir the syrup. Dot the mixture with the remaining 25g (2 tbsp) of butter. Allow the mixture to cool, not stirring, until the mixture registers at 50°C (122°F) - about 30 minutes. Be careful when taking the temperature, making sure not to agitate the mixture too much to prevent crystals from forming.
While the fudge syrup is cooling, take the time to prepare the pan. Grease a standard loaf pan with butter or cooking spray and arrange two pieces of foil, overlapping, to line the pan and serve as a sling to remove the fudge once set.
Using a hand mixer, a wooden spoon and some elbow grease or a stand mixer fitted with the paddle attachment, vigorously stir the syrup mixture until the consistency turns from shiny to matte in appearance - about 5-10 minutes. Note that this will be quicker if using a stand mixer or may take up to 15 minutes if stirring by hand. Stir in the peanut butter, if using.
Scrape the fudge mixture into the prepared loaf pan, smoothing the surface to ensure that it is even. Allow the fudge to set for at least 2 hours at room temperature. Use the foil to lift the fudge from the pan and then cut it into 24 equal squares.