Line an 8" x 8" pan with straight sides with parchment paper and set aside.

Place chocolate chips, sweetened condensed milk, and butter in a medium glass bowl and heat in the microwave for 1 minute. Stir well. If needed, heat 15 seconds more and then stir again, repeating until mixture is completely smooth. This could be done on the stovetop instead, using a double boiler.

Stir in the salt and vanilla.

Quickly yet gently, fold in the marshmallows and peanuts, taking care to not overwork or "smoosh" the marshmallows. You don't want the marshmallows to break down from the heat of the warm chocolate mixture.

Transfer mixture to prepared pan and spread into an even layer. If desired, press in additional peanuts and marshmallows over the top. Chill in the refrigerator for 1½ to 2 hours.

For easiest (neat & clean) cutting, gently lift the fudge out of the pan by the parchment paper edges and place it on a cutting board. Use a sharp non-serrated knife to cut into small pieces - I like to cut an 8" x 8" pan into 64 pieces, 8 cuts in each direction.