Lightly butter a 9- x 13-inch baking dish for thinner fudge or a 9×9 or 11×7 for thicker fudge.
In a medium saucepan over medium heat, combine butter, evaporated milk, and sugar; cook until mixture comes to a boil, stirring constantly. Boil until mixture barely reaches a softball stage (I use the cold water test) anywhere from 8-12 minutes, then remove from heat.
Stir in peanut butter, marshmallow cream, and vanilla and mix until thoroughly combined. Pour mixture into baking dish (do not scrape the pan!) and spread evenly. Cool and refrigerate until firm.
Cut into 1-inch squares. Store in an airtight container.
I always make my candy in an old pressure cooker pan. The bottom is heavy and thick and works so well to make candy in. If you have a heavy duty saucepan, I would definitely use that.
It usually takes anywhere from 7-12 minutes for fudge to reach a softball stage, the time depends on the weather, how high your stove is and what type of pan you are cooking it in, so just keep an eye on it.
One last thing, if you want a thicker fudge, use a 9×9 or 11×7-inch pan. I used a 9×13 and wish I would have used a smaller one so my fudge would have been thicker.