Line an 8 inch (20 cm) square pan with aluminum foil, leaving an overhang on at least two sides. Smooth out any wrinkles or creases.
Put the evaporated milk in a 4-quart (4 L) saucepan and fix a candy thermometer to the side.
Add the butter and sugar to the pan, and heat over medium-high heat, stirring frequently, until the temperature reaches 234˚F (112˚C).
Remove the pan from the heat and stir in the bittersweet chocolate, unsweetened chocolate, and marshmallow creme.
Stir in the nuts, then scrape the mixture into the prepared pan and smooth the top. Let cool at room temperature for at least 4 hours.
Once cool, lift the fudge from the pan and cut it into squares, whatever size you like.