Preheat the oven to 350 degrees. Grease a 9x13 inch pan with nonstick cooking spray. Set aside.
In a large bowl, mix together the butter, brown sugar and granulated sugar with a hand mixer until blended. Add in your eggs and vanilla extract until incorporated. Add in your peanut butter, and continue to beat until combined.
In a bowl, whisk together the salt and flour. Slowly add to your wet mixture and mix until just combined.
Spread into the prepared pan. Bake for 25 to 30 minutes until golden and a toothpick inserted in the center comes out clean. Remove the bars from the oven and let cool completely.
To make the frosting, in a large bowl beat together the butter and marshmallow fluff. Add in your powdered sugar 1 cup at a time until blended.
Spread the frosting on the bars, and sprinkle peanuts on top.