Place a large Dutch oven or other heavy-bottom pot on the stove over low heat. Add the butter and allow to melt. Add 10 oz of evaporated milk (if you only have a 12oz can, just don't use quite the whole thing).
Sift in the cocoa, using a whisk to incorporate fully.
Whisk in the powdered sugar, about 1 cup at a time. Be patient and keep whisking. This process will take several minutes.
Once the powdered sugar is completely incorporated, raise the heat to medium. Keep whisking and scraping down the sides as the mixture comes to a boil. This will take several minutes. Don't try to cut time by raising the heat - it is important to allow the sugar temperature to raise gradually.
Once the mixture starts to bubble, reduce the heat just a little and continue to whisk the mixture for about 6 more minutes. If you have a candy thermometer, you want to get just past 114℉ or just past the soft ball stage.
Turn off the heat. Quickly mix in a 12 oz jar of peanut butter. Remove the pot from the warm burner and quickly whisk in the marshmallow creme.
Transfer the mixture to a 9x13 baking dish lined with parchment paper. This is best done with two people. Work quickly and use a spatula to spread the fudge evenly and remove any bubbles.
Allow the fudge to rest undisturbed for at least an hour. Transfer to the refrigerator for at least 8 hours but ideally overnight.
Cut the fudge into squares and store in an airtight container in the refrigerator. It will last for months.