Line an 8x8 dish with parchment paper, allow for some overhang.
In a glass microwave safe bowl, combine the sweetened condensed milk, chocolate chips, and butter. Microwave 2 minutes on 50% power, stir, then microwave 30 additional seconds on 50% power. Add in the salt, vanilla, stir to combine. Save a small handful of marshmallows and peanuts for the top, otherwise add the rest. Stir to combine.
Pour the fudge into the pan, use a spatula to spread and smooth. Top the fudge with the leftover marshmallows and peanuts, pressing them into the top.
Allow the fudge to set on the counter for 3-4 hours. Cut into squares. If your fudge is slightly sticky, place in the fridge for 30 minutes before cutting. Store fudge in an air tight container on the counter.