1. Line an 8-inch (20cm) square pan with foil, leaving an overhang on at least two sides. Smooth out any wrinkles or creases.
2. Put the evaporated milk in a 4-quart (4l) saucepan and fix a candy thermometer to the side.
3. Add the evaporated milk, butter, and sugar to the pan, and heat – stirring frequently so it doesn’t burn – until the temperature reaches 234ºF (112ºC).
4. Remove the pan from the heat and stir in the bittersweet or semisweet chocolate, as well as the unsweetened chocolate and marshmallow cream.
5. Stir in the peanuts, then scrape the mixture into the foil-lined pan. Let cool for at least four hours.
Once cool, lift the fudge from the pan, and cut into cubes.