1. Butter an 8-inch square pan, set it aside. If you're giving the fudge for a gift, you may want to line the pan with foil and butter that, so you can lift the fudge out before cutting it.
2. In a heavy saucepan, combine the milk or cream, sugar, butter and salt. Heat over medium heat, stirring until the butter is melted and the sugar dissolves.
3. Continue heating until the mixture comes to a full, rolling boil; this is when the entire surface is covered with vigorously popping bubbles.
4. Boil without stirring for 8 to 10 minutes, or until the mixture registers 234° to 236°F on an instant-read or candy thermometer. (If you don't have a thermometer, see tips section on the right.)
5. When the fudge has reached the 234° to 236°F, immediately remove it from the heat, and stir in the marshmallows, peanut butter and vanilla until the mixture is smooth.
6. Pour the fudge into the prepared pan and allow it to cool overnight at room temperature, before cutting into 1-inch squares.