In a medium-size saucepan over medium-high heat mix the sugar and milk together. Bring to a boil and cook for exactly 8 minutes (this registers as the soft ball stage on a candy thermometer, if you are using one).
Remove the mixture from the heat and add the butter and marshmallow creme and stir until smooth. Add the chocolate and mix until melted (if the mixture looks "broken," beat with a wooden spoon or the paddle attachment of a stand mixer until smooth and fluid). Add in the marshmallows, peanuts, vanilla, and salt and mix until incorporated.
Pour into a greased 9-inch square pan. Cool completely in the refrigerator before removing from the pan and cutting into squares, 3 to 4 hours.